Preheat oven to 190 degrees C. Grease and flour one 20x30cm baking pan.
In medium bowl, cream the butter or margarine with the sugar until light. Beat in the eggs and root beer essence.
Combine the flour, baking powder and salt. Add the flour mixture to the egg mixture. Beat in the root beer and mix until just combined.
Pour batter into the prepared pan and bake for 30 to 35 minutes. Let cool then frost with icing.
To make icing: In a medium bowl beat the 125g butter or margarine, icing sugar, 1 teaspoon root beer essence and pinch salt. Beat in 1 to 3 tablespoons root beer to reach desired consistency. Spread onto cooled cake.