Ingredients:

2 c. brown sugar, firmly packed
1/2 c. butter

1/2 c. evaporated milk
1 tsp. vanilla

Directions:

Cream brown sugar, butter and milk in a saucepan.  
Place over moderate heat, stirring until sugar is dissolved.
Boil gently without stirring for 5 minutes, or, until mixture forms a soft ball when dropped in cold water.  
Remove from heat.  
Beat with an electric mixer until mixture has cooled & thickened to spreading consistency.  
Mix in the vanilla.  
Spread on cooled cake.