Combine the dry ingredients in a medium sized bowl: flour, Dutch cocoa powder, espresso powder, cornstarch, baking powder, and salt. Whisk and set aside.
1 3/4 cup all-purpose flour, spooned and leveled, 1/4 cup Dutch process cocoa, 2 tsp. espresso powder, 1 tsp. baking powder, 1 tsp. cornstarch, 1/2 tsp. salt
In a small saucepan over medium-low heat, melt the chocolate chips and butter. Stir just until ingredients are melted and be careful to watch so chocolate doesn't burn. Remove from heat, stir in vanilla extract and set aside to cool.
8 Tbsp. butter, unsalted, 1 cup chocolate chips (you can use milk, semi-sweet or dark), 2 tsp. vanilla extract
Using a stand-mixer fitted with the whisk attachment or a hand mixer, beat the eggs and sugars for 3-4 minutes or until thick and pale in color.
2 large eggs, room temperature, 3/4 cup brown sugar (light or dark), tightly packed, 1/4 cup white sugar
Using a rubber spatula, stir in the cooled chocolate/butter mixture until well combined.
Add in the flour mixture and fold until combined, avoid over-mixing. Dough will look shiny.
Chill cookie dough in fridge for at least 10 minutes so batter can thicken and look more like cookie dough. (Chilling the dough will also help the cookies bake up thicker & help prevent spreading). Preheat oven to 350°F while dough chills.
Scoop batter using a cookie scoop so cookies come out even in size and roll into a ball in the palms of your hands. Place on a baking sheet or cookie sheet lined with parchment paper or a silicone baking mat.
Partially bake the cookies for 8 minutes.
While the cookies bake, make the mocha glaze. Combine the powdered sugar with the melted butter, cocoa, espresso and salt. Stir until combined. Add a splash more coffee or water to get desired consistency.
1 1/2 cups powdered sugar, 2 1/2 Tbsp. butter, unsalted, melted, 2 1/2 Tbsp. espresso or strong hot coffee, 2 Tbsp. Dutch process cocoa, 1/4 tsp. salt
Remove the cookies from the oven and press half of a marshmallow, cut side down, into the center.
5 marshmallows, cut in half
Bake again for 2 additional minutes so the marshmallow can get soft. Remove from the oven and while the cookies are still warm, spoon some of the mocha glaze icing on top of each cookie spreading it around the cookie. Top with festive sprinkles. Allow the cookies to cool on cookie sheet for at least 5 minutes, then transfer to wire rack to cool completely. Enjoy!