Dark Chocolate Buttercream Frosting Recipe

Dark Chocolate Buttercream Frosting Recipe great for any cake.
Ingredients
12 ounces semi-sweet chocolate, chopped (chips are fine too!)
3 tablespoons dark cocoa powder (like Hershey's Special Dark 100% cacao)
2 cups (4 sticks) unsalted butter, softened (not melted, just soft, like a slightly ripe peach)
4+ tablespoons whole milk, divided
1 teaspoons pure vanilla extract
3 cups sifted confectioners powdered sugar (10x)
Source Link: http://shewearsmanyhats.com/2014/02/dark-chocolate-buttercream-frosting-recipe/