1/2 c. (1 stick) butter
1 c. firmly packed light brown sugar
1/4 c. milk
1/2 tsp. vanilla
2 c. (approximately) confectioners' sugar

Melt butter, add sugar, bring to boil. Boil over low heat for 2 minutes. Stir constantly. Add milk. Stir until this comes to a boil. Remove from heat, cool. Add vanilla. Slowly add confectioners' sugar, beating after each addition. Beat until thick enough to spread.