Dark brown sugar takes a long time to dissolve, so make sure it has completely dissolved before letting the mixture boil.

3 cups dark brown sugar, firmly packed
1 cup milk
1/2 ounce unsweetened chocolate, melted
1 tablespoon butter
1 (1-inch) piece vanilla bean, split lengthwise
1 1/2 cup coarsely chopped nuts

In a saucepan over medium heat, dissolve the sugar in the milk, and add the chocolate, stirring constantly. Reduce the heat and add the butter and vanilla bean. Bring the mixture slowly to a boil. Stir until the mixture reaches the soft-ball stage and remove the pan from the heat.

Discard the vanilla bean. Add the nuts, mixing them in thoroughly and beating until the candy begins to harden. Turn the candy out into a buttered 8-inch square pan. When it is completely cold, cut the candy into 1-inch squares.