Preheat your oven to 180°C (170°C fan/356°F) and grease two 8-inch round cake tins with butter or baking spray. In a large mixing bowl, combine the room-temperature eggs, caster sugar, self-raising flour, 3.5 oz custard powder, baking powder, milk, and salt. Beat with an electric mixer on medium speed for 2-3 minutes until the batter is smooth, pale, and well-combined—this aeration is crucial for a light, fluffy cake. Divide the batter evenly between the prepared tins and smooth the tops. Bake for 12-18 minutes until a toothpick inserted in the center comes out clean and the cakes are golden brown.

Remove the cakes from the oven and let them cool in the tins for 5 minutes, then turn them out onto a wire cooling rack to cool completely. Once fully cooled, use a serrated knife or cake leveler to gently slice off any domed tops—this creates flat, even surfaces for better assembly and a professional appearance. I like to do this gently with a sawing motion rather than pressing down, which helps keep the crumb structure intact.

While the cakes cool, prepare the frosting by adding the softened butter, sifted icing sugar, and 1.75 oz custard powder to a clean mixing bowl. Beat on medium speed for 3-4 minutes until the mixture is light, fluffy, and pale—this incorporates air and creates a smooth texture. If the frosting seems too thick to spread, add the tablespoon of milk a little at a time, beating well between additions. Finish by adding the vanilla extract and beat once more until fully incorporated.

Place one cooled cake layer on your serving plate or cake stand. Spread or pipe about half of the custard buttercream from Step 3 onto the top of the first layer in an even layer. Stir the rhubarb jam gently to loosen it slightly, then spread it evenly over the buttercream layer. I find that warming the jam very slightly (about 10 seconds in the microwave) makes it easier to spread smoothly without disturbing the frosting beneath.

Carefully place the second cooled cake layer on top of the jam, aligning it gently with the bottom layer. Spread or pipe the remaining custard buttercream from Step 3 onto the top of the cake, creating a smooth or decorative finish as desired. Dust the top generously with sifted icing sugar for a classic, elegant presentation. Slice and serve immediately—the cake is best enjoyed fresh, though it will keep well covered at room temperature for up to 2 days.