Yield: 12 cupcakes
Preheat oven to 350 degrees F. Line muffin tin with paper cupcake liners. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment (or using an electric hand mixer), cream together butter and sugar until smooth. Add in mashed banana, egg, and milk. Mix until combined. Scrape down bowl as needed.
In another bowl, whisk together flour, baking powder, baking soda, and salt. Add to mixing bowl and mix on low speed until there are no longer any dry streaks of flour. Scrape down sides of bowl as needed to ensure thorough mixing.
Fold in blueberries until evenly distributed. Divide batter among prepared muffin tin. Generously sprinkle granulated sugar over muffins.
Bake for 23-25 minutes until a toothpick inserted in center of muffin comes out clean. Allow muffins to cool in pan for 5 minutes before unmolding. Allow muffins to cool on wire rack. Serve muffins warm or at room temperature.