Preheat oven to 325°F. Spray a cupcake tin with nonstick spray or line 9 of the wells with paper liners. Set aside.
In a small saucepan, melt the butter and chocolate chips over medium-low heat. Once melted, set aside and allow to completely cool.
½ cup unsalted butter, ½ cup semisweet chocolate chips
In a medium-sized bowl, whisk together the eggs and sugar until combined. Whisk in the flour and vanilla. Pour the cooled chocolate mixture into the batter and fold it together with a spatula until the batter is combined and free of lumps. Scoop the batter into each of the cupcake liners, filling them about ¾ of the way full. A heaping amount in a large cookie scoop will give you the perfect amount, but you can spoon it in there as well (SEE NOTE).
2 large eggs, ¾ cup granulated sugar, ½ cup all-purpose flour, 1 teaspoon pure vanilla extract
Bake for 30-33 minutes, until a toothpick comes out clean. Allow the cupcakes to cool for 10 minutes before removing. Cool completely on a rack until ready to frost.
Make the Chocolate Buttercream (click for recipe) while brownie cupcakes cool.
½ batch Chocolate Buttercream
Frost the cooled cupcakes with the buttercream and top each with 1 rectangle of Hershey's chocolate.
1.55 ounces Hershey's milk chocolate