Preheat the oven to 350 degrees Fahrenheit. Line a baking sheet with parchment paper.
Add the dry ingredients; flour, cocoa powder, baking soda, and salt to a medium sized bowl and whisk them together until combined.1 1/4 cups All Purpose Flour, 1/3 cup Unsweetened Cocoa Powder, 1/2 tsp Baking Soda, 1/2 tsp Salt
Add the butter, and granulated sugar to a large mixing bowl (or the bowl of a stand mixer) and cream them together for about a minute or two until combined. 1/2 cup Unsalted Butter, 1 cup Granulated Sugar
Add in the egg, and vanilla extract and stir them in until just combined. 1 large Egg, 1 tsp Vanilla Extract
Slowly add in the dry ingredients while mixing until you have a nice, soft cookie dough.
Scoop the dough into 1 TBS portions, and roll them into round balls. Place on the prepared baking sheet, about 2-3 inches apart.
Bake for 7-8 minutes, then remove from the oven and immediately top each cookie with a half a marshmallow. 15 Large Marshmallows
Return the pan to the oven and bake for an additional 1-2 minutes, just until the marshmallow is soft, but not toasted.
Remove the pan from the oven and let the cookies cool on the pan for about 5 minutes, then transfer them to a cooling rack to cool completely.