Mix together the butter, sugar, eggs, baking powder, flour, and cocoa powder in a large bowl until the batter is smooth. Pour it into a springform cake pan and bake it at 350°F for one hour.
As soon as this layer is baked and has completely cooled off, spoon out a hole in the middle, setting that bit of cake carefully aside. Place the bananas side by side in the empty space in the center of the cake layer.
Whip the cream until it's stiff and fold in the chocolate shavings and vanilla extract. Then spoon it onto the bananas.
Crumble up the part of the cake you spooned out and sprinkle the crumbs over the top.