In a microwave-safe bowl, melt the butter and chocolate chips in the microwave. Start off with 1 minute on high, then stir well. If all chocolate chips aren’t melted yet, continue to heat in 30-second intervals until melted. Stir well after each round in the microwave, as you want these just melted but not too hot.
½ cup unsalted butter, 1 cup semisweet chocolate chips
In a separate large bowl, using a hand mixer, beat the melted butter/chocolate mixture with the brown sugar until well mixed, about 1 minute.
1½ cups brown sugar
Beat in the eggs and vanilla just until combined. Make sure the brown sugar and chocolate mixture is cool before adding the eggs so you don’t accidentally scramble the eggs. Set aside.
2 large eggs, 2 teaspoons pure vanilla extract
In a separate bowl, whisk the hot cocoa powder, flour, cornstarch, baking powder, and salt together.
½ cup hot cocoa powder, 1¾ cups all-purpose flour, ½ tablespoon cornstarch, 1½ teaspoons baking powder, ¼ teaspoon kosher salt
Pour the dry ingredients into the wet ingredients and beat just until mixed. Do not overmix!
Fold in all of the mini chocolate chips.
1 cup mini semisweet chocolate chips
Use a 2-tablespoon-sized cookie scoop to scoop the cookies onto a parchment-lined baking sheet. Place the cookies close together (but not touching) so that they all fit onto 1 baking sheet to fit into the freezer.
Place the baking sheet of cookies in the freezer for 10 minutes. This is an important step so the cookies don’t spread too much during baking.
While the cookies chill, preheat oven to 350°F.
Now that dough balls are slightly frozen and less sticky, use your hands to shape these into round balls.
Line a separate baking sheet with parchment paper and place some cookies from the freezer onto this new baking sheet. The cookies should be spaced at least 2 inches apart.
Bake the cookies for 10-12 minutes, or until the cookie edges seem set (tops will still look slightly wet, but they’ll finish baking on tray after removing from oven). Let the cookies cool completely on the baking sheet before transferring them to a wire rack to cool completely.
Once the cookies have cooled, place the marshmallow fluff into a piping bag and pipe it over the cookies.
1 cup marshmallow fluff