In a heavy, medium skillet place 1/3 cup sugar. Cook over medium-high heat until sugar begins to melt (do not stir); shake skillet occasionally to heat sugar evenly. Reduce heat to low and cook until sugar is melted and golden brown; stir as necessary after sugar begins to melt. Pour caramelized sugar into an 8-inch quiche dish. Tilt dish to coat bottom. Let stand 10 minutes.
Combine eggs, milk, 1/3 cup sugar, and vanilla. Beat until well combined. Place quiche dish in a 13x9x2-inch baking pan on the oven rack. Pour egg mixture into prepared quiche dish. Pour boiling water into the baking pan around quiche dish to a depth of 3/4 inch. Bake in a 325 degree F oven for 20 to 25 minutes or until a knife inserted near the center comes out clean.
Carefully lift quiche dish from water. Transfer to a wire rack. Serve warm or cool completely and cover and chill up to 24 hours. Loosen edge of dish with a spatula or knife, slipping point of spatula down sides to let air in. Invert onto a serving platter. Garnish with mango slices or other fresh fruit. Makes 6 servings.