Preheat oven to 350 degrees. In a mixing bowl, whisk together flour, baking powder, baking soda and salt, set aside. In a separate mixing bowl, whisk together mashed banana and sour cream, set aside.
In a large mixing bowl, using an electric hand mixer on medium-high speed, whip together butter, granulated sugar and light-brown sugar until pale and fluffy, about 3-4 minutes. Blend in egg. Add banana mixture and mix until combine. Slowly add dry ingredients and mix just until combine.
Scoop dough by rounded tablespoon fulls (about 1 1/2 Tbsp) and drop onto Silpat or parchment lined baking sheets placing them about 1 1/2 inches apart, then smooth edges and flatten cookie as needed (diameter of cookie should be about 1 1/2 - 2 inches.
Alternately you can fill a pastry bag fitted with a 1/4 inch plain tip with batter, then pipe dough into 1 1/2 inch rounds onto baking sheet - if you do this method you may get more than 2 dozen pies). Bake in preheated oven for 12 - 13 minutes until edges are golden.
Allow to cool 2 minutes on baking sheet then transfer to a wire rack to cool completely. Spread and sandwich a layer of frosting between two cookies. Store in an airtight container (in refrigerator if not serving within a few hours) .
Fluffy Vanilla Bean Frosting
In a large mixing bowl, using an electric hand mixer on medium high speed, whip butter and vanilla bean seeds until pale and fluffy, about 4 minutes. Add powdered sugar, milk and vanilla extract and blend 2 minutes until light and fluffy.