Step 1: Prepare the Dough

1. In a large mixing bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy.
2. Beat in the egg and vanilla extract, mixing until smooth.

Step 2: Mix the Dry Ingredients

1. In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
2. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Step 3: Incorporate Marshmallows

1. Gently fold in the mini marshmallows, being careful not to overmix.
2. Cover the dough with plastic wrap and refrigerate for 30 minutes to firm up.

Step 4: Preheat and Prepare

1. Preheat your oven to 350°F (175°C).
2. Line baking sheets with parchment paper or silicone baking mats.

Step 5: Shape and Bake

1. Scoop 1 1/2-tablespoon portions of dough and roll them into balls. Place them on the prepared baking sheets, leaving about 2 inches between each cookie.
2. Bake for 10–12 minutes, or until the edges are set but the centers are still soft.
3. Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

Step 6: Decorate (Optional)

1. Drizzle melted chocolate over the cookies and sprinkle with flaky sea salt for a finishing touch.