In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, sugar, and brown sugar together on high speed until smooth and creamy, about 2 minutes.
½ cup unsalted butter, ⅓ cup granulated sugar, 2 tablespoons brown sugar
Add the egg and egg yolk and beat together to incorporate, about 30 seconds.
1 large egg, 1 egg yolk
Add the flour, cocoa powder, milk, baking soda, baking powder, and salt. Mix until combined, about 30 seconds.
1 cup all-purpose flour, ¼ cup unsweetened cocoa powder, 1 tablespoon milk, ½ teaspoon baking soda, ¼ teaspoon baking powder, ½ teaspoon kosher salt
Tightly wrap the dough in plastic wrap and refrigerate for at least 45 minutes (or overnight).
Once the cookie dough has chilled, remove the plastic wrap. Preheat oven to 350°F. Line a baking sheet with parchment paper. Set aside.
Add 1 cup of coarse sugar to a small bowl. Set aside.
1 cup coarse sugar
Scoop 1 tablespoon of dough per cookie and roll into balls. Roll each cookie dough ball in the coarse sugar until completely coated.
Place the cookie dough balls onto the prepared baking sheet with at least 2-3 inches of space between each cookie.
Bake the cookies for 10-12 minutes, or until the edges are set.
Transfer baked cookies to a cooling rack to cool completely.
Place the pink melting wafers into a microwave safe bowl. Heat in 30 second increments, stirring between each.
½ cup pink melting wafers
Pour the melted wafers into a Ziplock bag. Cut a small hole into one corner of the bag.
Drizzle each cookie with the melted chocolate and top with sprinkles (if desired) before the chocolate hardens.
½ cup Heart-shaped sprinkles