Make the frosting. In a medium bowl, add the butter, cream cheese, lemon zest and powdered icing sugar. Using a hand-held electric beater, cream the frosting for 3 minutes until light and airy. Add in one tablespoon of lemon juice at a time, making sure to beat well after each addition, until the desired consistency is reached. If the icing is too thick, then add more lemon juice and mix thoroughly. If the icing is too runny, then thicken it up with ¼ cup of powdered icing sugar at a time.