Preheat the oven to 350 degrees. Line a cupcake pan and set aside.
In the bowl of a stand mixer, combine the butter, sugar, salt, and baking powder. Cream until smooth.
Add in the milk, banana, vanilla, and eggs, and mix well.
Add the flour and mix on medium speed for two minutes.
Fill each cupcake liner until about ⅔ full. Bake in preheated oven for approximately 14-16 minutes or until edges are slightly browned and a toothpick comes out clean. Cool completely.
Frosting
Cook the berries over low in a small saucepan until warm and juicy. Press the mixture through a fine mesh sieve. Cool completely.
In the bowl of a stand mixer, cream the butter until smooth.
Add in the powdered sugar a cup at a time, alternating with the berry puree.
Mix on low speed for 30 seconds and then medium speed for about a minute. Stop the machine, scrape down the sides and mix again. If the frosting appears too lumpy and dry, add a tablespoon of milk, only if needed. Mix again.
Transfer the frosting to a piping bag fitted with a large star tip. Pipe a mound of frosting on each cooled cupcake. Garnish with fresh berries and banana slices.