Preheat the oven to 350° F and line two baking sheets with parchment paper.
Melt the butter, add the hot melted butter to the bowl of a stand mixer. Add the cocoa powder and espresso powder and mix until smooth and well incorporated.
1 cup butter, 1/2 cup cocoa powder, 1 Tablespoon instant espresso powder
Add the sugar, brown sugar, eggs, and vanilla to the chocolate mixture and mix until fully combined.
1 cup sugar, 3/4 cup brown sugar, 1 egg, 1 1/2 teaspoons vanilla extract, 1 egg yolk
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
2 1/4 cups flour, 1 teaspoon baking powder, 1/4 teaspoon baking soda, 1 teaspoon salt
Add the dry ingredients to the wet ingredients and mix until just combined and no streaks of flour remain.
Cover the dough and refrigerate for 30 minutes to firm up.
Scoop the dough into portions (about 2 tablespoons each), roll into balls, and place on the prepared baking sheets about 2 inches apart.
Gently flatten each ball with the bottom of a glass to create an even surface.
Bake for 10–12 minutes, until the cookies are set. Let cool on a wire rack.
While the cookies cool, heat the cream until hot, but not boiling, and pour over chocolate chips, do not stir. Let sit for about 10 minutes, then stir gently until smooth to create the icing.
1/3 cup heavy cream, 1 cup semi-sweet chocolate chips
Once cookies are completely cool, spoon icing onto each cookie and spread evenly. Top with rainbow sprinkles and let set.
Rainbow sprinkles