Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.

In a large mixing bowl, whisk together flour, sugar, cocoa, baking soda, and baking powder.

In a separate bowl, whisk together milk, water, oil, egg, and root beer extract.

Pour the wet ingredients into the dry ingredients and beat on medium speed for 3 minutes.

Divide the batter evenly among the prepared cupcake liners.

Bake for 10-12 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cupcakes cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely.

While the cupcakes are cooling, prepare the frosting by whipping together powdered sugar, root beer extract, water, and shortening until smooth and creamy.

Once the cupcakes are completely cool, frost them with the root beer frosting.