Rhubarb and Custard Crumble Bars

A colleague once brought these to a staff meeting, and I spent the entire presentation thinking about them. The way the tart rhubarb cut through the sweet custard, all under that buttery crumble, felt like spring in a single bite. I asked for the recipe on the spot, scribbled it on the back of an agenda, and made them that weekend. They've been my go-to ever since when I want something impressive that doesn't require pastry skills.

I made these for a spring picnic once, and they survived a bumpy car ride better than I expected. Everyone kept asking if I'd bought them somewhere, which is the highest compliment a home baker can get. One friend even requested them for her birthday instead of cake, which made me realize these bars had become more than just a recipe I borrowed.

Ingredients

• Fresh rhubarb: Choose firm, bright stalks and trim away any leaves, they're toxic, and the tangier the rhubarb, the better the contrast with the custard.
• Granulated sugar (rhubarb layer): This tames the rhubarb's sharpness without making it cloying, and the cornstarch helps it set into a jammy layer that won't leak.
• Lemon juice: Just a tablespoon brightens the rhubarb and keeps the flavor lively, not flat.
• Cornstarch (rhubarb layer): Essential for thickening the fruit so it doesn't turn your bars soggy, don't skip it.
• All-purpose flour: Forms the structure of both the base and the crumble, giving you that classic shortbread texture.
• Rolled oats: These add a rustic crunch to the crumble topping and make it feel a bit more wholesome.
• Unsalted butter: Cold and cubed is key, it creates those little pockets of flakiness that make the crumble golden and crumbly.
• Light brown sugar: Adds a subtle molasses note that deepens the flavor of the crumble without overpowering the fruit.
• Salt: A small amount balances the sweetness and makes every other flavor pop.
• Whole milk: The fat content makes the custard rich and smooth, skim milk won't give you the same creamy result.
• Egg yolks: These thicken the custard and give it that pale yellow color and velvety texture.
• Granulated sugar (custard layer): Sweetens the custard just enough to complement the tart rhubarb without competing with it.
• Cornstarch (custard layer): Stabilizes the custard so it sets properly when baked and doesn't weep or separate.
• Vanilla extract: A teaspoon is enough to make the custard taste like the kind you remember from childhood desserts.

Instructions

Prep your pan:

Preheat your oven to 180°C (350°F) and line a 23cm square baking pan with parchment paper, leaving an overhang on two sides. This little trick makes lifting the bars out later almost foolproof.

Cook the rhubarb:

Combine the chopped rhubarb, sugar, lemon juice, and cornstarch in a saucepan and cook over medium heat, stirring often, until the rhubarb breaks down and the mixture thickens, about 8 to 10 minutes. Set it aside to cool while you work on the base.

Make the crumble mixture:

In a large bowl, mix the flour, oats, brown sugar, and salt, then rub in the cold butter with your fingertips or a pastry cutter until it looks like coarse breadcrumbs. The butter should stay in little bits, not turn into a paste.

Form the base:

Press two-thirds of the crumble mixture firmly into the bottom of your prepared pan to create an even layer. Reserve the rest for the topping and don't worry if it feels a bit crumbly, it will hold together once baked.

Bake the base:

Bake the base for 15 minutes until it's lightly golden around the edges. Remove it from the oven and let it cool slightly while you make the custard.

Prepare the custard:

Heat the milk in a saucepan until it's steaming but not boiling, then whisk together the egg yolks, sugar, and cornstarch in a bowl until smooth. Gradually pour the hot milk into the egg mixture, whisking constantly, then return everything to the pan and cook over medium heat, stirring, until it thickens, about 2 to 3 minutes, then stir in the vanilla.

Layer the bars:

Spread the cooled rhubarb evenly over the baked base, then pour the custard over the rhubarb in a smooth layer. The custard will settle and find its level, so don't overthink it.

Add the crumble topping:

Sprinkle the reserved crumble mixture evenly over the custard layer, covering as much surface as you can. It's fine if a few gaps show through.

Bake until golden:

Bake for 30 minutes, or until the crumble is golden brown and the custard is just set with a slight wobble in the center. It will firm up as it cools.

Cool and chill:

Let the bars cool completely in the pan, then transfer to the refrigerator and chill for at least 2 hours before lifting out and slicing. This step is non-negotiable if you want clean cuts.

Save

The first time I served these at a garden party, someone asked if they were difficult to make. I said no, and they didn't believe me until I walked them through it step by step. That's when I realized these bars have a magic trick built in, they look and taste like effort, but they're mostly just patience and good timing.

How to Know When They're Done

The crumble should be deeply golden, not pale, and the custard should have a gentle jiggle in the center when you shake the pan. If the custard looks loose or liquid, give it another 5 minutes. I learned this after pulling a batch too early and ending up with a delicious mess that had to be eaten with a spoon.

Storing and Serving

These bars keep beautifully in the fridge for up to three days, covered loosely with foil or plastic wrap. I like them cold, straight from the fridge, but some people prefer them at room temperature with a dusting of powdered sugar. A scoop of vanilla ice cream on the side never hurts, especially if you're serving them as dessert instead of a teatime treat.

Variations and Tweaks

If rhubarb isn't your thing, or you can't find it, swap in the same amount of chopped strawberries or a mix of berries. The custard layer is forgiving enough to work with almost any tart fruit. Once, I added a handful of raspberries to the rhubarb on a whim, and the extra tartness made the bars even more interesting.

• Try adding a pinch of ground ginger or cardamom to the crumble for a subtle spice note.
• Swap half the oats for chopped almonds if you want a nuttier crumble topping.
• For a richer custard, use half-and-half instead of whole milk, though it will be a bit heavier.

Save

These bars have become my answer to spring, a season that always feels too short. They capture that fleeting moment when rhubarb appears at the market and the air smells like rain and new grass, and every time I make them, I'm reminded why I borrowed that recipe in the first place.

Recipe Questions & Answers

→ Can I use frozen rhubarb instead of fresh?

Yes, frozen rhubarb works well. Thaw it first and drain excess liquid before cooking to prevent the mixture from becoming too watery.

→ Why do the bars need to be chilled before cutting?

Chilling helps the custard layer set properly and makes cutting clean, neat bars much easier. At least 2 hours in the refrigerator is recommended.

→ Can I make these bars ahead of time?

Absolutely. These bars are ideal for making ahead. They store well in the refrigerator for up to 3 days and actually improve in texture after chilling overnight.

→ What can I substitute for rhubarb?

Tart fruits like cranberries, sour cherries, or a mix of rhubarb and strawberries work wonderfully. Adjust sugar levels to taste depending on fruit acidity.

→ How do I know when the custard layer is properly set?

The custard should be just set but still have a slight jiggle in the center when you gently shake the pan. It will firm up further as it cools.

→ Can I freeze these bars?

While possible, freezing may affect the custard texture. If freezing, wrap individual bars tightly and consume within one month. Thaw in the refrigerator overnight.