Add flour, brown sugar, cinnamon and cold diced butter to a bowl. Using a pastry cutter cut into the mixture until it all comes together and you have pea-size crumbles (largest) down to small crumbs. Set aside.
Add bananas to another bowl and smash.
Add sugar, melted butter, egg and vanilla to the bananas and mix.
Add flour, baking soda and salt to the banana mixture. Stir.
Grease donut molds and place on a foil or parchment lined cookie sheet. Fill with 1/4 C of batter. Top with the streusel topping.
Bake at 350 for 20 minutes.
Cool for 10 minutes and remove from the molds. Drizzle in melted butter for extra decadence.