In a medium bowl, beat together the peanut butter, granulated sugar, brown sugar, vanilla and egg white.

In a small bowl, stir in the oat flour, baking powder and salt. Add the oat mixture into the peanut butter mixture and stir until well combined and a sticky dough forms.

Cover and refrigerate the dough for at least 2 hours.

Preheat oven to 350°F and spray a mini muffin tin with cooking spray.

Once the dough is chilled, press good 1/2 tablespoon balls into the each mini muffin cup. Mold the dough so that it's pressed up along the sides and hollow in the middle, like a little cup. You want the sides a little bit thicker than the bottom of the cup.

Bake until golden brown, about 10-11 mins.

When the cups come out of the oven, they will have puffed up and lost some of their shape, quickly (without burning your finger tips off!) press the centers down, and in, slightly to restore their shape. Let cool completely.

While the cups cool, you can make the meringue. In a large bowl, beat the egg whites until foamy. Gradually add the granulated sugar and cream of tartar and keep beating until soft peaks form.

Press one sliced banana in the bottom of each cookie cup, and then cover it with peanut butter, filling right to the top of the cookie cup. This is about 1 teaspoon of peanut butter.

Spoon meringue on top of each cookie cup (or you can use a piping bag to make it less messy, and prettier) and then place the cookie cups on a large, ungreased baking sheet.

Place back into the 350°F oven and bake until the meringue is golden brown, about 10 minutes.

Drizzle with melted chocolate, if desired, and serve.