Preheat oven to 350 degrees Fahrenheit. Line 2 baking sheets with parchment paper.
Combine and mix flour, baking powder, baking soda, and salt in a medium bowl.
In a separate large bowl, beat softened butter and both sugars (brown and regular) with an electric mixer on medium-high speed for about 2-3 minutes until light in color and creamy. Add egg and vanilla, continuing to beat until well-incorporated. Add mashed bananas and Greek yogurt and beat just enough to combine.
Add flour mixture to the wet ingredients and mix until combined.
Drop dough by heaping teaspoonfuls onto the prepared baking sheets. Bake for 10-12 minutes until the edges just begin to brown. Let cookies cool by slide the parchment papers with cookies on them onto the wire rack.
How to make peanut butter frosting:
In a medium bowl, using electric mixer, beat softened unsalted butter and peanut butter until creamy.
Add the sugar and continue beating. When the frosting starts to thicken, add a tablespoon of milk at a time and continue beating for several more minutes to reach the desired consistency: so that the frosting is fluffy, has the right kind of thickness and can spread easily.
Spoon 1 tablespoon of peanut butter frosting onto the flat sides of half the cookies. Sandwich with remaining cookies. Dust with confectioners' sugar, and/or sprinkle with sprinkles as shown on photos, if desired!