Mix the butter, eggs, vanilla essence and sugar together. Add the self-raising flour and custard powder then add the milk. Beat well for 10 minutes. Grease a baking dish as this makes a large slab of cake.
Mix the cinnamon, sugar, flour and butter until it resembles bread crumbs, then sprinkle over the top of the cake. Bake for 45-50 minutes at 180 degrees C.