Place the cake mix, the egg, butter or margarine in a bowl and knead well, or beat for around 2 minutes with a hand mixer or electric mixer on low speed until the mixture takes on a crumb consistency. Press 2/3 of the crumbs into the base of a 26 cm diameter greased baking tin.
Spread the chosen filling onto the crumb base and cover the filling with the remaining dough crumbs (1/3 of the mixture).
Place the baking tin on the lowest shelf of the preheated oven and bake for around 50 minutes.