Step 1: Cream the softened butter with both sugars for 2 minutes. Beat in the egg and vanilla until smooth.

Step 2: Whisk the flour, cocoa, baking soda and salt in a separate bowl. Mix into the butter mixture just until no dry streaks remain. Fold in the chocolate chips.

Step 3: Cover and chill the dough for 30 minutes. Heat the oven to 350°F (175°C) and line two baking sheets with parchment.

Step 4: Scoop about 1 1/2 tablespoons of dough per cookie. Flatten each scoop, place half a marshmallow in the middle and seal the dough around it, leaving a little marshmallow visible at the top.

Step 5: Arrange cookies 2 inches apart. Bake for 9–11 minutes until the edges look set but centers remain soft. Let cool on the tray for 5 minutes before transferring to a rack.