In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt; then set aside.

In a large bowl, beat the softened butter with both granulated and brown sugars until light and fluffy using an electric mixer.

Add eggs one at a time, mixing well after each; then stir in the vanilla extract.

Gradually add the dry mixture to the wet ingredients, mixing on low speed until just combined, being careful not to overmix, and fold in chocolate chips if using.

Cover and refrigerate the dough for at least 30 minutes to 2 hours to meld flavors and prevent spreading.

Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.

Scoop rounded tablespoons of dough, optionally roll them in powdered sugar, and place them 2 inches apart on the prepared baking sheets.

Bake for 8-10 minutes until the edges are set but the centers remain soft.

Remove the baking sheets and quickly place 3-4 mini marshmallows on top of each cookie, pressing gently.

Return the cookies to the oven for 2-3 minutes more, until marshmallows are puffed, lightly golden, and cookies are set.

Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely, allowing the marshmallows to set.