Preheat oven to 375° F. Line two baking sheets with parchment paper and set aside.

In a small bowl, whisk or sift together the flour, baking soda, baking powder, and salt; set aside. In a large bowl, cream together the butter, 1/2 cup granulated sugar and the brown sugar until creamy. Stir in the peanut butter, egg and vanilla and beat until well combined. Add in the dry ingredients and mix until just combined. Cover with plastic wrap and chill the dough for at least 1 hour.

Shape into 1-inch balls and roll around in the remaining granulated sugar. Place on the prepared baking sheets about 2 inches apart. Use a fork to press the cookies down lightly and create a criss cross pattern. Bake until lightly golden brown around the edges, about 10 to 12 minutes. Allow to cool slightly on the baking sheets before transferring to wire racks to cool completely. Continue baking off the cookies until the dough is all used up.

To make the banana buttercream, beat the butter on medium speed just to get it going. Add in the mashed banana, vanilla and salt and mix until well combined. Add the sugar, a cup at a time, and beat on high until thick and creamy. Transfer to a piping bag with a plain circle tip.

Once the cookies have cooled, pipe out frosting on half of them and then sandwich together with the remaining cookies. Allow to set about 10 minutes before eating, either in the fridge or at room temperature. Leftovers can be wrapped in plastic and stored at room temperature for up to 3 days. Enjoy!