• Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper. Set aside.
• In a large mixing bowl, use a hand mixer or stand mixer to cream together the peanut butter, unsalted butter, granulated sugar, and brown sugar until smooth and fluffy (about 2 minutes).
• Add the egg and vanilla extract, mixing until fully incorporated.
• In a separate bowl, whisk together the flour, baking powder, and salt.
• Gradually add the dry ingredients to the wet ingredients, mixing until a soft dough forms.
• Using a 1-tablespoon scoop, drop cookie dough onto the lined baking sheet about 2 inches apart. Use a fork to gently press each dough ball, creating a crisscross pattern. This helps flatten the cookies and gives them the classic peanut butter cookie look.
• Bake for 10-12 minutes, or until the edges are golden and the tops look set. Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

Assembly

• Once the cookies are fully cooled, spoon about 1 tablespoon of marshmallow fluff onto the bottom of one cookie.
• Drizzle 1 teaspoon of peanut butter evenly over the marshmallow fluff.
• In a microwave-safe bowl, melt the chocolate chips in 20-second intervals, stirring between each until smooth. Drizzle or spread the melted chocolate over the peanut butter.
• Place a second cookie on top, pressing gently to create a sandwich.
• Let the sandwiches set for 10-15 minutes before serving. For firmer chocolate, chill them in the refrigerator for 10 minutes.