Preheat the oven to 350℉. Line a baking sheet with parchment paper; set aside.
Using a large mixing bowl and electric hand mixer or a stand mixer fitted with the paddle attachment, combine the butter and brown sugar on high speed until smooth and creamy.
Add the eggs and vanilla, then mix on medium speed to incorporate.
To a separate bowl, add the flour, cocoa, salt, and baking powder, and whisk well to combine.
With the mixer running on low speed, add the flour mixture, a little at a time, to the butter/sugar mixture. Stop the mixer and scrape down the insides of the bowl between each addition if needed.
Using a 2-tablespoon cookie scoop, portion balls of cookie dough 1-2 inches apart on the prepared baking sheet
Bake the cookies in preheated 350 ℉ oven for 10 minutes, or until they've puffed up and are slightly firm on the top.
Remove the pan from the oven. Using the back side of a small spoon, quickly make an indent in the top of each cookie. Place 3 mini marshmallows in the indentation. Return pan to the oven and continue baking for 2-3 minutes, until marshmallows are slightly melted. Do not overbake!
Notes
Avoid over baking. Because of their dark color, it is easy to overbake chocolate cookies. Remove them from the oven when the edges are set and the tops are firm, but not fully set.
Optional Toppings
Top with broken bits of peppermint candy canes, white chocolate chips, M&Ms, or even crushed pecans or walnuts.
Mallow bits: Substitute the mini marshmallows with the crunchy mallow bits that come in a hot chocolate packet. These little crunchy bits are also sold in packages on their own!
S'mores! Crush or break graham crackers into small pieces and sprinkle them over the melted marshmallows for a winter variation of s'mores cookies.