Preheat the oven to 350 degrees. Line two muffin tins with cupcake liners.
In a large bowl combine the butter, sugar, and brown sugar. Mix until combined.
Add in the eggs and mix well.
In a medium bowl combine the flour. cocoa powder, baking soda, baking powder, and salt.
Add half of the dry ingredients to the butter mixture and mix well.
Add in the root beer and mix until you have a smooth batter.
Add the remaining flour mixture and mix until combined.
Fill the muffin cups 3/4 of the way full. Bake for 22-24 minutes. Remove from oven and cool completely.
To make the glaze combine the powdered sugar and root beer in a small bowl. Whisk until smooth.
Spoon 1-2 teaspoons of the glaze on top of each cupcakes and spread around until it soaks into the cupcakes.
To make the frosting place the butter in a large bowl and beat on medium speed until fluffy, about 2 minutes.
Add the powdered sugar, a cup at a time, until the mixture is fluffy.
Add the vanilla, root beer, and salt, and beat for 2 minutes.
Pipe the frosting onto the cooled cupcakes.
Top with a cherry and a third of a paper straw if desired.