Preheat the oven to 350°F and line two large baking sheets with parchment paper.
In a large bowl, add peanut butter, butter, and sugars. Using a hand mixer, combine the mixture until smooth and creamy, about 2‐3 minutes.
Add egg and vanilla and mix until just combined.
Mix in flour, baking powder, baking soda, cornstarch, salt and mix until combined.
Using a 2-tablespoon cookie scoop, drop dough onto the baking sheets. Flatten each dough ball into a flat disk about 3-inches across.
Scoop 1 tablespoon of marshmallow fluff to the center of each disk of dough.
Bring the cookie dough up around the marshmallow fluff, completely covering the fluff and forming a ball.
Place on the lined baking sheets.
Press a few peanut butter chips into the ball dough
Bake the cookies for 9-10 minutes, or until the edges are brown
Remove from oven to wire rack and allow to cool completely