Preheat oven to 350 F. Line a 12 cup muffin pan with cupcake liners and set aside. In a large bowl, whisk together flour, sugar, salt and baking powder.
Add in vegetable oil, root beer, eggs and vanilla extract. Whisk until smooth. Evenly divide cupcake batter between the prepared muffin pan. Bake for 20-25 minutes, let cool completely.
For the root beer buttercream frosting, in a large bowl, beat softened butter and powdered sugar together. Add in root beer and vanilla extract, beating until smooth. Pipe or spread frosting onto cooled cupcakes.