In a medium-size heavy bottomed saucepan bring the brown sugar and heavy cream to a boil. Lower the heat and simmer stirring until the sugar has dissolved. Set aside to cool enough to still be pourable but, not set.
In a medium-size mixing bowl using an electric mixer on medium speed, whip the butter and vanilla with the brown sugar cream, Stop to scrape the sides of the bowl periodically.
Lower the speed, of the mixer and gradually add the confectioners sugar. Stop and scrape the sides of the bowl between additions.
Add additional heavy cream 1 Tbsp at a time to thin to spreading consistency, as needed.
After the powdered sugar is added, raise speed to medium and beat until light and fluffy.
Store chilled in an airtight container until ready to use. Beat again before using.
Refrigerate any leftovers.
Frosts: 24 cupcakes, 1 9-inch (2) layer cake or (1) 9 x 13-inch sheet cake