Cream the butter and sugars in the bowl of a stand mixer with a paddle attachment, or in a large bowl with a hand mixer, at medium-high speed until light and fluffy. Beat in the eggs and vanilla.
In a medium bowl, whisk flour, cocoa mix, baking cocoa, salt, baking soda and baking powder. Add the flour mixture to the wet ingredients and mix until combined.
Use a two tablespoon cookie scoop to drop cookie dough balls about two inches apart onto a prepared baking sheet lined with parchment paper. Bake at 375 degrees for 10-12 minutes. Remove from the oven and use the back of a measuring cup to flatten the tops of the cookies. Allow them to rest on the baking sheet for 1-2 minutes before transferring to a wire rack to cool completely.
Hot Chocolate Glaze: Whisk together the powdered sugar, remaining hot cocoa mix, and milk. Spread over the top of each cookie and add 3-4 mini marshmallows.