Make the streusel by combining dry ingredients together. Work the butter into the dry mixture until its the consistency of wet sand. To make the filling, whisk together the brown sugar, cinnamon and cocoa powder until very well combined.
In a stand mixer with whisk attachment (or by hand), beat butter and sugar until smooth and well combined. Add eggs, beating well after each one until well combined. Add mashed bananas and vanilla extract, beating until well combined.
Bake at 350F for 45 minutes. Check the topping. Return to the oven for 10-15 minutes, checking for doneness every 5 minutes (55-65 minutes total). Cool in pan for 20 minutes. Gently remove from the spring form pan. Cool an addtional 30-45 minutes.