Add the brown sugar, cream, and butter to a saucepan. Bring to a low boil over medium heat, stirring to prevent sticking, then boil for one minute.
1 cup brown sugar, ¼ cup whipping cream, 2 teaspoons butter
Remove from the heat and add the icing sugar and vanilla. Beat with an electric mixer for up to one minute, or until smooth and thicker. If the frosting seizes and looks grainy, mix in a splash more of cream.
1 cup sifted icing sugar, 1 teaspoon vanilla extract
Frost your cooled cake immediately. Keep at room temperature for a day or so, then refrigerate for up to a week. This frosting will set and form a thin outer shell.