Preheat oven to 350°F (175°C). Grease a 9-inch round cake pan.
In a bowl, whisk together flour, baking powder, baking soda, salt, and espresso powder.
Beat butter and sugar until fluffy, then add mashed bananas, eggs, and vanilla.
Alternate adding dry ingredients and sour cream to the mixture. Stir in milk.
Pour batter into pan and spread evenly.
Mix topping ingredients and sprinkle over the batter.
Bake for 40 minutes or until a toothpick comes out clean.
Cool in the pan for 10 minutes, then transfer to a rack to finish cooling.