Frosted Chocolate Cookies

Preheat the oven to 375°F and line a large baking sheet with parchment paper and set aside.
In a large mixing bowl, whisk together 4 cups all-purpose flour, ½ cup unsweetened cocoa powder, ½ cup black cocoa powder, 2 tablespoons powdered milk, 2 tablespoons cornstarch, and 4 teaspoons baking powder. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream together 1½ cups salted butter (cold and cubed), 1¼ cup dark brown sugar, 1 cup granulated sugar, and 1 teaspoon salt.
Add 2 large eggs, ¼ cup heavy cream, and 1 tablespoon vanilla extract and mix until thoroughly incorporated, about 1 minute, scraping down the sides of the bowl halfway through.
Add the dry ingredients to the wet ingredients in three separate additions, mixing between each one, until the ingredients are mostly incorporated.
Stir in the chopped 4 ounces milk chocolate and use a #8 cookie scoop to portion out the dough into 5-ounce portions. Roll the dough in your hands to create large balls, then place four at a time on the baking sheet and gently press them down.
Bake for 11 to 13 minutes, rotating halfway through. When done, the tops of the dough should no longer have a glossy appearance. Remove from the oven and allow to cool completely before frosting.

When ready to frost, add ½ cup unsalted butter, 2½ cups powdered sugar, ¼ cup unsweetened cocoa powder, and 1 tablespoon vanilla extract to a large mixing bowl and beat until smooth and pale.
Add 4 tablespoons heavy cream and beat until light and fluffy (about 2 minutes. Add up to 2 additional tablespoons of heavy cream to get the desired consistency.
Add a #24 scoop of frosting (about 2 tablespoons) to the top of each cookie and use an offset spatula to frost the cookies.
Top with chocolate sprinkles and coarse sugar if desired before the frosting crusts.