Equipment: 12 cup standard muffin tin
Preheat the oven to 350 degrees F. Line standard muffin tins with cupcake liners or spray footed cupcake holders with nonstick spray.
Put the butter, root beer, and cocoa in a microwave-safe bowl, cover with plastic wrap, and microwave on HIGH until the butter melts, about 2 minutes. Whisk to combine.
Meanwhile, whisk the flour, sugar, baking powder, baking soda, and salt together in a large bowl. Whisk the sour cream, eggs, and vanilla together in a small bowl.
Whisk the hot cocoa mixture into the dry ingredients. Stir in the sour cream mixture just to combine; take care not to overmix. Divide the batter evenly between the prepared muffin cups, filling each one about 3/4 of the way full, about 1/4 cup per cupcake. Bake cupcakes until a toothpick inserted in the center of the cake comes out clean, about 25 minutes. Cool the cupcakes completely in the tin on a rack. Remove from liners and return cakes to a rack.
For Frosting: Bring a few inches of water to a boil in a saucepan that can hold a mixer's bowl above the water. Whisk the egg whites, sugar, cream of tartar, and salt in the bowl by hand. Set the bowl above the boiling water and continue whisking until the mixture is hot to the touch and the sugar dissolves, about 1 to 2 minutes. Transfer the mixture to a standing mixer fitted with the whisk attachment and beat at medium-high speed until eggs beat up into a cool stiff meringue, about 5 minutes. Add a few drops of desired extract and/or food coloring and lightly beat into the meringue.
To decorate: Using a spoon, offset spatula or piping bag, dollop, spread or pipe about 1/2 cup meringue on top of each cupcake to make a ghost or monster of your choice. Decorate with candied eyeballs or other candies. These can be finished 2 to 3 hours in advance and held at room temperature.