First Step: Preheat your oven to 350 degrees and line a baking sheet with parchment paper. In a mixing bowl, whisk together 2 cups all-purpose flour, 1/2 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/2 teaspoon salt, then set aside.

Second Step: In another bowl, whisk together 1 cup mashed overripe bananas and 1/4 cup sour cream. Using an electric hand mixer on medium-high speed, beat 1/2 cup softened butter, 1/2 cup granulated sugar, and 1/2 cup packed light-brown sugar until pale and fluffy, about 3-4 minutes.

Third Step: Add 1 egg and mix well, then blend in the banana mixture. Slowly add the dry ingredients and mix just until combined.

Fourth Step: Scoop the dough by rounded tablespoonfuls (about 1 1/2 tablespoons) onto the prepared baking sheets, spaced 1 1/2 inches apart. Smooth the edges and flatten the cookies to about 1 1/2 to 2 inches in diameter.

Fifth Step: Bake for 12 to 13 minutes until the edges are golden. Cool on the baking sheet for 2 minutes, then transfer to a wire rack to cool completely.

Sixth Step: For the fluffy vanilla bean frosting, use an electric hand mixer on medium-high speed to whip 1 cup butter at room temperature and seeds from two small vanilla beans until pale and fluffy, about 4 minutes. Add 4 cups powdered sugar, 2 1/2 tablespoons milk, and 1 teaspoon vanilla extract, then mix for 2 minutes until light and fluffy.

Seventh Step: Once cooled, spread a layer of frosting between two cookies to form a sandwich. Store in an airtight container and refrigerate if not serving within a few hours. Remember, the total preparation time is about 1 hour and 10 minutes, yielding around 24 servings.