Preheat oven to 350°F (175°C). Line a baking sheet with parchment.
Cream peanut butter, butter, and sugars until fluffy (2–3 min). Add egg and vanilla.
In another bowl, whisk flour, baking powder, and salt.
Gradually mix dry ingredients into the wet until a soft dough forms.
Scoop 1-tbsp portions, space 2 inches apart. Flatten with fork crisscross.
Bake 10–12 min until edges are golden. Cool 5 min, then move to rack.
Melt chocolate in 20-sec intervals until smooth.
On one cookie, add 1 tbsp marshmallow fluff, 1 tsp peanut butter, then chocolate.
Top with another cookie and press gently. Let set 10–15 min or chill 10 min.
Serve and enjoy!
