1Preheat oven to 375 degrees F (190 degrees C). Grease and flour one 8x12 inch baking pan.
2In medium bowl, cream the butter or margarine with the sugar until light. Beat in the eggs and root beer extract.
3Combine the flour, baking powder and salt. Add the flour mixture to the egg mixture. Beat in the root beer and mix until just combined.
4Pour batter into the prepared pan and bake at 375 degrees F (175 degrees C) for 30 to 35 minutes. Let cool then frost with frosting.
5TO MAKE FROSTING: In a medium bowl beat the 1/2 cup butter or margarine, confectioner's, 1 teaspoon root beer extract and pinch salt. Beat in 1 to 3 tablespoons root beer soda to reach desired consistency. Spread onto cooled cake.