I found this recipe on the Martha Stewart website and it was AMAZING! They just melt in your mouth.
Banana Whoopie Pies
2 cups all-purpose flour
1/2 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1/2 cup mashed banana (from 1 large ripe banana)
1/2 cup sour cream
4 ounces (1 stick) unsalted butter, softened
1/2 cup granulated sugar
1/2 cup packed light-brown sugar
1 large egg
1/2 teaspoon pure vanilla extract
8 oz. ounces cream cheese, softened
1/4 c. butter, softened
2 1/2 cups confectioners' sugar, plus more for dusting
1 tsp vanilla extract
2 Tbs milk
Preheat oven to 350 degrees. Line 2 baking sheets with parchment. Sift flour, baking powder, baking soda, and salt into a bowl. Combine banana and sour cream in another bowl.
Beat butter and granulated and brown sugars with a mixer on medium-high speed, until pale and creamy, about 3 minutes. Add egg and 1/2 teaspoon vanilla, beating until incorporated. Add banana mixture in 2 additions, alternating with flour mixture.
Transfer batter to a pastry bag fitted with a 1/4-inch plain tip. Pipe batter into 1 1/4-inch rounds on baking sheets, spacing rounds 1 1/2 inches apart OR use an extra small (about tablespoon size) cookie scoop to drop dough on parchment paper.
Bake until puffed up and toothpick comes out clean, the perfect time for my oven was 9 1/2 minutes. You don't want them to brown on top, then they will be dry.
Beat cream cheese and butter together until combined. Add confectioners' sugar, 1/2 teaspoon vanilla, and milk with a mixer on medium speed until smooth, about 2 minutes. This should be consistency of frosting, adjust with more powdered sugar or milk if needed.
Transfer cream cheese filling to gallon size zip top bag, snip of corner and pipe filling onto cookies and then sandwich together. Best if eaten the same day, I like dusting with powdered sugar because they can be a little sticky.
Makes about 2 dozen whoopie pies.