01 -
In a bowl, stir together flour, cornstarch, baking powder, baking soda, and salt. Keep it to the side.
02 -
Using a mixer, cream together the softened butter and peanut butter until smooth. Toss in the egg, both types of sugar, and vanilla, then whip it up until it's airy.
03 -
Gently fold in the dry mix with the butter mixture. Chill in the fridge for half an hour.
04 -
Roll two tablespoons of dough into a flat round. Put a tablespoon of marshmallow fluff at the center, then fold the dough around it to seal.
05 -
Pop dough balls onto a baking sheet lined with parchment and bake at 350°F for around 10 minutes, or until they're set.