1. Combine brown sugar and butter in a small, heavy bottomed saucepan. Cook and stir over medium heat until butter melts and sugar is dissolved. 2. Stir in heavy cream and bring mixture to a low, gentle boil and cook one minute,. 3. remove from heat. Add vanilla and whisk. Then add confectioners sugar and whisk until smooth. Cool until lukewarm before drizzling over the cake. (Glaze will thicken as it cools. It should be thick but still pourable.)