Preheat oven to 350 F. Line a cupcake pan with 12 cupcake liners. In a large bowl, whisk together flour, sugar, salt and baking powder.
Add in vegetable oil, milk, eggs and vanilla extract. Whisk until smooth. Divide cupcake batter into three separate bowls. Add red food coloring into one bowl, mixing until fully red. Add blue food coloring into another bowl, mixing until fully blue.
Layer cupcake batter as red, white and blue into the prepared cupcake tins. Swirl with a knife. Bake for 20-25 minutes. Let cool completely.
For the vanilla buttercream frosting, in a large bowl in either a hand or stand mixer, beat softened butter with powdered sugar, milk and vanilla extract until smooth. Add in more milk as needed to thin. Frost and decorate cooled cupcakes.
