In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt. Set aside.

In a large mixing bowl, beat butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes.

Add eggs one at a time, mixing thoroughly after each addition, then blend in vanilla extract.

Gradually add dry ingredient mixture to wet ingredients on low speed until just mixed.

If sticky, chill dough covered in the refrigerator for about 30 minutes.

Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper. Shape cookie dough into balls.

Press marshmallows or marshmallow fluff onto the top of each cookie dough ball.

Bake for 10-12 minutes until edges are set but centers remain soft.

Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.