In a saucepan, combine heavy cream and brown sugar. Heat until sugar dissolves. Remove from heat. In a stand mixer, Add egg yolks and beat until light and frothy. Slowly add 3/4 of the cream mixture to the yolks. Add the egg mixture to the remaining cream and whisk. Add the sweet potato mixture, vanilla, and spices. Whisk until smooth. Strain the mixture.
Heat oven to 325 degrees. Place 4 ramekins in a baking pan. Divide mixture between them. Pour boiling water in the baking pan until it reaches half way up the ramekins. Bake for 25-30 minutes or until centers are just set. Remove from water bath and cool for 10 minutes. Refrigerate for 4 hours or overnight.
Sprinkle 1 tablespoon of sugar over each custard. Use a torch to caramelize the tops.