In a small saucepan, melt butter and chocolate chips over low heat. Stir until combined and smooth. Set aside to cool for about 10 minutes.
In a medium bowl, whisk together flour, hot cocoa, baking powder, and salt. Set aside.
In the bowl of a stand mixer, beat the brown sugar and eggs together. Add vanilla extract. Slowly add in dry ingredients. Mix in the melted chocolate. Refrigerate cookie dough for at least 1 hour or overnight.
Preheat oven to 325 degrees F. Line a large baking sheet with a Silpat baking mat or parchment paper. Using about a teaspoon of dough, form cookie dough balls and place on prepared baking sheet.
Bake for 6 minutes, or until cookie tops begin to crack. Pull cookies out from oven and place 1 mini marshmallow in the center of each cookie. Place a few Mallow Bits on each cookie as well. Bake cookies for an additional 4 minutes, or until marshmallows are soft and cookies are set around the edges, but still soft. Let the cookies cool on the baking sheet for 2 minutes. Transfer cookies to a wire cooling rack and cool completely.
Note-I buy the Mallow Bits at Target. If you can't find them, just use the mini marshmallows. They will still be tasty:)