Cream butter, granulated sugar, and brown sugar for 2-3 minutes until fluffy. Add eggs and vanilla; mix until smooth.

In another bowl, whisk together flour, cocoa powder, baking soda, and salt. Combine wet and dry ingredients until just mixed.

Fold in chocolate chips and walnuts until evenly distributed. Chill dough for 30 minutes.

Preheat oven to 350°F (175°C). Scoop dough onto parchment-lined baking sheet, leaving space between each.

Bake for 8 minutes, then press mini marshmallows on top and return to oven for 2-3 minutes until marshmallows puff.

Cool for 5 minutes on baking sheet, then transfer to wire rack.