Cream the butter and sugar together.
Add the eggs one at a time, mixing well after each addition.
Sift in the cake flour and baking powder, then mix into a batter.
Spread the batter evenly into the pan.
Mix the crumble ingredients (butter, flour, sugar) to make small lumps and sprinkle them on top of the batter.
Bake at 170°C for 35-40 minutes.