How to Make It
Preheat the oven to 350° and line 2 baking sheets with parchment paper. In a large bowl, whisk the 1 1/2 cups of the flour with the baking powder, salt, cinnamon and espresso powder.
In a small bowl, whisk the egg with the granulated sugar, bananas, walnuts, melted butter and vanilla. Using a wooden spoon, stir in the dry ingredients.
In a small bowl, using a fork, combine the remaining 2 tablespoons of flour with the brown sugar and the chilled butter cubes until coarse crumbs form.
Using a large ice cream scoop, scoop 4 mounds of the batter onto each baking sheet. Sprinkle the batter with the crumbs and bake in the upper and lower thirds of the oven for about 14 minutes, until golden and a toothpick inserted in the center comes out clean; shift the pans from top to bottom and front to back halfway through baking. Let the muffin tops cool slightly before serving.