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.
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.