In a medium bowl, mix together the pureed sweet potatoes, 1/4 cup of the sugar, and the lemon juice until combined. Butter 8 8-ounce custard cups or ramekins. Spoon 2 to 3 tablepoons of the sweet potato mixture into each cup to form a 1/4-inch layer.

Preheat the oven to 325 degrees F.

In a medium saucepan, combine the heavy cream, vanilla bean, and 3/4 cup of the remaining sugar. Bring the mixture to a boil over medium heat. Remove the pan from the heat.

In a large bowl, whisk together the egg yolks and another 3/4 cup sugar. Gradually pour the egg mixture into the saucepan, whisking constantly. Return the saucepan to the stove and cook on low heat for 3 minutes, or until the mixture thickens.

Fill the custard cups with enough of the cream mixture so they are about seven-eighths full. Place the filled cups in a baking pan that is large enough to hold them all. Add enough hot water to come within 1 inch of the top of the cups. Bake for 45 minutes, or until the custards are barely set and a toothpick inserted in the center comes out a little wet. Remove the custards from the baking pan and place them in the refrigerator. Refrigerate overnight.

Preheat the broiler. Lightly sprinkle the surface of each custard with 1 1/2 tablespoons of the remaining sugar. Place the custards under the broiler for about 30 to 60 seconds and let them brown. Keep an eye on the custards, as this happens quickly.

Remove the custards from the heat, and once the sugar has hardened (1 to 2 minutes), serve the creme brulee.