Pre-heat an oven to 180°C/ 350°F. Greece a 6-inch spring foam pan and set aside. Mix all the ingredients for the crumb topping and set aside. Whisk together flour, baking soda, baking powder and salt together in a bowl.
Puree plantains till smooth with no lumps. Add butter/ margarine and sugar and mix together. Add vanilla essence and butter milk and mix till blended.
Add in the dry ingredients and mix till incorporated. Do not over mix. Pour the batter into the prepared pan and sprinkle the prepared crumbs on top.
Bake in the pre-heated oven for 29 to 34 minutes (my cake is usually done on the 31st minute, do check early) or till a tooth pick inserted comes out clean.
When choosing plantains for this cake, use a black spotted and soft one. It should be ripe, soft from the normal ripe ones but not over ripe that is totally black. Use pastry flour if you in Ghana, it is the same as all purpose flour. As with most baked goods, the flavour of this cake intensifies the next day, you can make ahead for an intense flavour. Sweet plantain crumb cake can be stored at room temperature for 3 days or the freezer for 3 months. Thaw in the fridge, slice and serve when ready.