|dark brown sugar, preferably Muscovado|
In a small saucepan, place the brown sugar. With a small sharp knife, split the vanilla bean lengthwise in half. Scrape the vanilla bean seeds into the sugar. Rub the seeds in with your fingers. Remove and reserve the pod. With a silicone spatula, stir in the butter.
Bring the mixture to a boil over medium heat, stirring constantly. Remove the pan from the heat and stir in the heavy cre