In a mixing bowl, cream together the brown sugar, butter, and egg.
In a separate bowl, whisk together the flour and baking powder.
Stir the root beer extract into the water.
Add the dry ingredients to the creamed mixture, alternating with the root beer water.
Mix until just combined.
Drop by rounded tablespoons onto a parchment lined baking sheet and bake for 8-10 minutes. (Helpful tip: The cookie is like a soft sugar cookie.)
Beat the icing ingredients to a spreadable consistency, and frost the cookies once they are completely cooled.
Notes
*Details on the root beer extract and where to buy are included in the main recipe post. Prefer a cream cheese frosting? Mix together 1 cup (120g) powdered sugar, 4 tablespoons melted butter, 4 ounces cream cheese (softened), 1½-2 teaspoons root beer extract, and ½ teaspoon vanilla extract.