Rhubarb and Apple Custard Crumble Cake

Rhubarb and Apple Custard Crumble Cake

• 23 Jun 2025

Winter suddenly arrived in Sydney so it's time to make comfort food. I had some leftover oven roasted rhubarb in my fridge and a few green apples in the fruit bowl, so I made a rhubarb and apple custard crumble cake. 

I've made an apple custard teacake before and a rhubarb version, so I was quietly confident that a rhubarb and apple cake topped with crumble would be a winner. The base of the cake comes from Sift by Nicola Lamb; the caramelised apples from Nicola Lamb's Substack, Kitchen Project; the custard from this Women's Weekly recipe and the oat crumble was adapted from a Nigel Slater recipe. There will be a bit too much fruit for the top but it keeps well and you can use it to top your morning porridge.

Here's the recipe for you which makes either an 8-inch round or square cake. For all my recipes I use a 250ml cup and a 20-ml tablespoon, unsalted butter and 60g eggs. My oven is a conventional gas oven so if your oven is fan forced you may need to reduce the oven temperature by 20°C.

Rhubarb and Apple Custard Crumble Cake

Vanilla custard
2 tablespoon custard powder
40g caster sugar
1 cup (250ml) milk
20g butter
2 tsp vanilla extract

Crumble
25g rolled oats 
50g plain flour
40g raw sugar 
Pinch of salt
Pinch cinnamon
50g unsalted butter

Oven roasted rhubarb
200g rhubarb
Zest of ½ orange
35g caster sugar
30g water or orange juice

Caramelised Apples
300g Granny Smith apples (you'll need 200g prepared weight)
50g caster sugar
25g unsalted butter

Cake
120g room temperature unsalted butter

60g caster sugar
60g light brown sugar
1 tsp vanilla
½ tsp flaky salt 
2 eggs
110g plain flour
50g spelt flour
1 tsp baking powder
¼ tsp bicarb soda
120g sour cream or Greek yoghurt

Custard
Combine the custard powder and sugar in a small saucepan; gradually stir in the milk. Stir over heat until the mixture boils and thickens. Remove from heat; stir in the butter and essence. Press plastic wrap over the surface; cool.

Crumble
Stir together the oats, flour, sugar, salt and cinnamon. Rub the cold butter into the mixture until crumbs form. You want a mix of smaller and chunkier crumbs. Set aside or keep in the fridge until ready to use.

Oven roasted rhubarb
Preheat the oven to 190°C, conventional. Cut the rhubarb into 8cm x 2cm batons, then mix with the orange zest, sugar and water or orange juice in a small roasting tray. Roast for 15-17 minutes or until the rhubarb is slightly tender. Leave to cool overnight in the fridge for the best colour. If you don't mind that too much, then you can use it right away.

Caramelised apples
Peel and core and cut into even pieces, about 1-2cm in width. In a low, wide frying pan, add the caster sugar. Heat the sugar over medium-high heat until it starts to melt, caramelize, and even burn in places, about 3-4 minutes. You can move it around if you want, but I normally just let it do its thing. Add the butter along with the apples, reduce the heat to medium-low and let the mixture come to bubble. Lower the heat, then allow the apples to simmer in the caramel sauce for about 10 minutes or until tender. Depending on the varieties, you might get a mixture of mushier and firmer pieces. Once the apples are caramelised and softened, remove the pan from heat. Let the caramelised apples cool slightly before using, or cool completely and store in the fridge for 3 days.

The cake
Preheat the oven to 190°C, conventional. Grease, flour and line the base of the tin with baking paper. If using a square tin then line the tin with baking paper that extends above the lip of the tin by 5cm to help you remove it later. 
 
Cream the butter and sugars together for 2–3 minutes until light.  You don’t need to go ultra-white and fluffy here. Next, emulsify in the eggs. Due to the ratio between butter and eggs, it WILL probably look (and be) split. Don’t worry. Continue anyway. Stir through the sifted flours, salt and raising agents. Finally, stir through the sour cream.

Spread the batter evenly on the base. Loosen the custard by stirring or blitz with a stick blender if lumpy. Dot the cake generously with blobs of custard, then place the apples and rhubarb all over the top. You may not have room for all the fruit. Finally, scatter with the crumble, making sure there is still space for the cake and custard to peek out. 

If making the round cake bake for about 1 hour to 1 hour and 10 minutes. If baking the square cake, it will take less time to bake so start checking after 45 minutes. It’s ready when there’s only the slightest hint of a wobble in the middle. A skewer may not come out clean because of the fruit and custard so check the internal temp is 96°C. 

Cool in the tin for about 30 minutes before carefully removing the cake and leaving it to cool completely on a rack for about 2 hours before cutting. Leftovers can be stored in the fridge for 3-4 days but bring to room temperature before serving. 

As expected this rhubarb, apple and custard cake topped with crumble was a delight and will definitely be added to my winter repertoire.

See you all again next week with some more baking from my kitchen.

Bye for now,

Jillian