Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a large mixing bowl, beat together peanut butter, unsalted butter, granulated sugar, and light brown sugar until light and fluffy.

Add egg and vanilla extract to the mixture and mix until well combined.

In a separate bowl, whisk together flour, baking powder, and salt. Gradually add to wet ingredients until a soft dough forms.

Scoop out dough, roll into balls, and place on prepared baking sheet. Flatten each ball with a fork.

Bake for 10-12 minutes until edges are golden brown. Cool on sheet for 5 minutes before transferring to a wire rack.

Melt milk chocolate chips in a microwave-safe bowl until smooth.

Spread marshmallow fluff and creamy peanut butter on one cookie, drizzle with melted chocolate, and top with another cookie.

Let the cookies set at room temperature for 10-15 minutes or refrigerate for about 10 minutes to firm the filling.