• Preheat oven to 350°F degrees. Line a large baking pan with a silicone baking mat or parchment paper. Set aside.
• In a medium bowl, combine flour, baking powder, baking soda, and salt. Set aside.
• In a small bowl, combine mashed banana and Greek yogurt until almost smooth. Set aside.
• In the bowl of a stand mixer, cream butter, granulated sugar and  brown sugar until pale and fluffy. Add egg and vanilla extract, and mix to combine.
• With the mixer on low speed, gradually add dry ingredients and banana-yogurt mixture, alternating between dry and wet until just combined.
• With an ice cream scoop or piping bag fitted with a plain round tip, scoop or pipe batter into approximately 1½ inch rounds, leaving about 2 inches between each.
• Bake for 12-14 minutes or until set and just beginning to brown around the edges. Remove from baking sheet and place on cooling rack until completely cool.

For Filling:

• Combine butter, marshmallow fluff, vanilla extract, and confectioners' sugar in the bowl of a stand mixer. Beat until smooth.
• Spread or pipe in the center of half the cakes. Top each with a like-sized cake, and squeeze gently until filling comes to edges.
• Dust with powdered sugar before serving.