Combine mashed sweet potatoes, 1/4 cup brown sugar, and lemon juice; spoon potato mixture into a buttered 10-inch quiche dish to form a 1/4-inch-thick layer.
Stir together cream, 1 cup sugar, egg yolks, and vanilla in medium saucepan. Cook over low heat, stirring constantly, about 5 minutes or until hot. Pour over sweet potato mixture in prepared dish. Place dish in a shallow baking pan. Add hot water to pan to a depth of 1 inch.
Bake at 325° for 1 hour or until knife inserted in center comes out almost clean. Remove from water. Cool on a wire rack. Cover and refrigerate at least 8 hours.
Sprinkle custard with 1/3 cup brown sugar; place custard on jellyroll pan. Broil 5 1/2 inches from heat about 2 minutes or until sugar melts. Let stand 5 minutes to allow sugar to harden before serving. Garnish, if desired.