In a large mixing bowl, mix the flour and baking powder for the batter. Set bowl aside.
In a medium mixing bowl, add the melted butter and mashed bananas, mix well. Add in the sugar, eggs, vanilla extract and milk and mix well with a hand mixer.
Slowly combine the liquid ingredients into the dry ingredients. Set aside.
In a medium mixing bowl, add in the ingredients for the crumb topping. Cut the butter into the other ingredients with a fork until you have a crumbly mixture.
Using a casserole crock-pot, spray the inside of the crock with cooking oil.
Pour in half the batter and top with half the crumb topping mixture. Pour the remaining batter on top of this and then top with the remaining crumb topping.
Cover and cook on HIGH for 2 hours or until the middle has set and the edges are golden brown.
Remove the crock and allow to cool.
Slice and serve just slightly warm.