To make this blueberry magic cake also known as a custard cake, which forms a fudgy base, custard filling, and sponge cake top as it bakes, pre-heat your oven to 160C Fan or equivalent, and grease and line a 9-inch square cake tin. Beat together the egg yolks and icing sugar until thick and very pale in colour. Add in the zest and juice of half a lemon, followed by the flour, and then the melted butter. Beat together until well combined. Gradually add in the milk, mixing well between each addition. Take care as this makes the mixture very runny.
In a separate bowl whisk the egg whites for a few minutes, then add in the sugar. Keep whisking until the mixture holds firm peaks when the whisk is lifted out, this may take a while. Gently fold in the egg whites, 1/3 at a time, into the milky mixture, until everything is combined. There will be quite a few lumps in the mixture, but that’s perfectly fine. Pour this in to your square cake tin and bake in the oven for 1 hour, until the cake is golden brown on top and wobbles slightly. Leave in the tin to completely cool down as this will allow the cake to set, then cut into squares. As you lift the pieces out of the tin you will see that the three layers have formed inside the cake making it truly magical. Sprinkle over a few more blueberries and dust with icing sugar to serve, delicious!