1. Freeze mini marshmallows for 20 minutes. Optional for easy stuffing: Pipe or spoon 1-teaspoon dollops of marshmallow fluff onto a parchment-lined tray and freeze 20–30 minutes. Preheat oven to 350°F (175°C). Line 2 baking sheets with parchment.

2. In a large bowl, beat softened butter, peanut butter, granulated sugar, and brown sugar until light and fluffy, 2–3 minutes.

3. Beat in egg and vanilla until smooth and glossy.

4. Whisk flour, baking soda, baking powder, and salt. Add to the wet ingredients and mix on low just until combined. Stir in peanut butter chips, if using.

5. For stuffed cookies: Scoop 2 tablespoons (35–40 g) dough, flatten into a disk, place a frozen fluff dollop or 3–4 frozen mini marshmallows in the center, then pinch and roll to seal. Arrange 2 inches apart on the sheets.

6. For swirled cookies: Dollop about 1/3 cup fluff over the finished dough and fold 2–3 times to marble—do not fully mix. Scoop dough mounds (2 tablespoons each) and place on sheets.

7. Bake 8–10 minutes, until edges are set and tops look puffed with pale centers. If marshmallow leaks, use a spoon to nudge it back while warm. Tap the pan once or twice for gentle crinkles.

8. Cool on the sheet for 10 minutes, then transfer to a rack to finish cooling. Enjoy warm and gooey or at room temperature.