These soft and moist banana bread cupcakes topped with a crunchy and buttery streusel are mini versions of your classic favorite.
Preheat oven to 350 F. Line a muffin pan with paper cups. In a bowl, beat together softened butter and brown sugar until it is creamy and fluffy. Beat in the egg until incorporated. Add the mashed bananas and vanilla and beat to combine it with the rest of the ingredients.
In a bowl, whisk together flour, baking powder, baking soda and salt. Add this flour mixture to the mixing bowl, alternately with the buttermilk, making three additions of the flour mixture and two additions of the buttermilk. Beat the mixture just until blended. Fold in the chocolate chips. Spoon mixture into the prepared muffin pan until they are 2/3 full. Sprinkle the streusel on top of each cupcake batter. Bake for 20-25 minutes, or until a toothpick inserted at the center of a cupcake comes out clean with some bits of moist crumbs.
Combine all streusel ingredients in a medium bowl. Using a pastry cutter or your fingers, cut the butter ( or rub, if you are using your hands) into the rest of the ingredients until the mixture is powdery and coarse.