Streusel: mix all the ingredients together until there are moist streusel.
Stir the butter / margarine, sugar and eggs until creamy. Add the warm milk, flour, salt and finally the baking powder and spread the mixture into a greased springform pan, pulling the edge up a little.
Cover the dough with the ripe bananas - either cut lengthways or in small wheels. Put the finished crumble on top and bake the cake in a preheated oven at 180-200 degrees / 35-45 minutes until golden brown.