Preheat your oven to 350°F (175°C). In a bowl, whisk together the flour, baking powder, baking soda, and salt. In a separate large mixing bowl, cream the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla and mashed bananas. Add the dry ingredients alternately with the buttermilk, mixing just until incorporated.

Spoon the batter into the lined muffin tin, filling each cup about two-thirds full. Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center of a cupcake comes out clean. Allow the cupcakes to cool in the tin for about 5 minutes before transferring to a wire rack to cool completely.

While the cupcakes are cooling, prepare the buttercream. In a large bowl, beat the softened butter until creamy. Gradually add the powdered sugar, one cup at a time, mixing well after each addition. Add the mashed bananas, vanilla extract, and a pinch of salt, beating until the frosting is light and fluffy.

Once the cupcakes are completely cool, frost them generously with the banana buttercream using a piping bag or spatula. Decorate with banana slices or sprinkles as desired.

Store the frosted cupcakes in an airtight container at room temperature for up to 2 days, or refrigerate for up to 5 days. For longer storage, freeze the unfrosted cupcakes and thaw before decorating and serving.