Preheat the oven to 325 degrees F. Arrange 6 4-oz ramekins in a 9×13 baking dish.
In a saucepan, combine the cream, brown sugar, and 1/4 cup sugar. Bring to a simmer over medium high, and stir to make sure the sugar dissolves. Take the pan off the heat once that sugar has dissolved.
In a large bowl, whisk the egg yolks until they get frothy and light yellow. Okay, time to temper. Whisking constantly so the eggs don’t scramble, add 3/4 cup of the hot cream to the frothy eggs. Now add the egg mixture to the remaining hot cream, and keep whisking constantly. Add the vanilla, cinnamon, nutmeg, and sweet potatoes, and whisk until smooth and combined.
Set a fine mesh sieve over a large bowl and strain the creme brulee mixture. Quick note: there will be some fibers in the sweet potatoes that won’t want to go through the sieve. If you want somewhat of a silky smooth custardy texture, don’t force the sweet potato through the sieve. Believe me, it will already impart plenty of sweet potato flavor, even with some of those fibers left behind. Pour the custard into the ramekins.
Create a bain marie by pouring boiling water halfway up the sides of the ramekins. We always cook custards in a water bath to moderate the cooking temperature and ensure that the custard never reaches a temperature higher than 212. Bake until the custards are barely set in the center, but still jiggly, about 45 minutes. Remove from the water bath and refrigerate for at least 3 hours, until thoroughly chilled.
Sprinkle each custard with 1/2 tsp sugar, and caramelize it with a blow torch, the broiler in your oven, or an iron. Enjoy!