First, make the custard by mixing together the custard powder and caster sugar in a small saucepan. Whisk in the milk and bring the pan to the boil, stirring frequently. The mixture will start to thicken very quickly and, when it does, take the pan off the heat. Whisk in the butter and vanilla extract. Place some clingfilm directly onto the surface of the custard to prevent a skin from forming and leave to cool.
Preheat the oven to 180°C (350°F).
Cream the butter and sugar until it is light and fluffy.
Beat in one egg at a time, together with a tablespoon of the flour to help it all come together.
Add the remaining flour, baking powder and custard powder, and mix well. The batter will be quite thick because there is no liquid, but it should still be spreadable.
Line the bottom of a 20 cm (8 inch) springform cake tin with baking paper and grease the sides of the tin.
Use a small spatula to spread half of the batter in the cake tin. Remember that it is a thick batter so you will have to make sure that it covers all of the surface.
Spread the custard over the cake mixture.
Dollop spoonfuls of the remaining cake batter over the custard and carefully spread the cake batter to completely cover the custard. Again, a small spatula works well here.
Chop the rhubarb into 10 cm lengths and slice them lengthwise, about 1 cm thick.
Arrange the rhubarb as in the photo, trimming them where necessary.
Brush the top of the cake with melted butter and sprinkle generously with granulated sugar.
Bake for about 1 1/2 hour. Remove the cake from the tin and cool on a wire rack.