This frosting is best when used immediately but can be stored in the refrigerator, covered with plastic wrap, for up to two days. Stir frosting until smooth before using.
1. In a double boiler, melt butter. Remove from heat and stir in confectioners' sugar, milk, salt, lemon extract, and lemon juice. Return to heat and cook, stirring occasionally, for another 5 minutes. Do not melt the sugar.
2. Remove from heat; beat until cool and thick.