1Heat oven to 325°F. Butter 10-inch quiche dish.* In medium bowl, mix mashed sweet potato, 1/4 cup brown sugar and the lemon juice. Spoon mixture into quiche dish.
2In 2-quart saucepan, stir together whipping cream, granulated sugar, egg yolks and vanilla. Cook over medium-low heat about 15 minutes, stirring frequently, until hot (do not boil). Pour over sweet potato mixture. Place dish in shallow pan. Place pan in oven. Pour enough boiling water into pan to depth of 3/4 inch (about halfway up side of dish).
3Bake 1 hour or until knife inserted in center comes out almost clean. Carefully remove dish from water. Cool on cooling rack. Cover; refrigerate at least 8 hours.
4Set oven control to broil. Sprinkle custard with 1/3 cup brown sugar; place dish in 15x10x1-inch pan. Broil with top 4 to 6 inches from heat 3 to 5 minutes or until sugar is melted. Let stand 5 minutes before serving.