Preheat the oven to 350 degrees F (175 degrees C).

Whisk the egg in a medium saucepan, and mix in the 1 lemon's zest and juice, 1/4 cup sugar and cornstarch. Set over medium heat, and cook stirring constantly until the mixture thickens. Reduce heat to low, and continue whisking for another minute. Remove from the heat and stir in the butter. Divide between four 6 or 8 ounce ramekins. Set aside.

In a medium glass or metal bowl, whip egg whites with an electric mixer. When they are able to hold a soft peak, sprinkle in 1 tablespoon of the sugar, and continue mixing until stiff. Whisk the remaining 4 tablespoons of sugar into the egg yolks along with the zest and juice of the remaining lemon. Fold a couple of spoonfuls of the egg whites into the yolks to lighten them up, then fold in the rest of the whites. Spoon into the ramekins over the lemon curd, and run a finger around the inside of each rim.

Place the ramekins onto a baking sheet, and place in the preheated oven. Bake for 15 to 17 minutes, until puffed and golden brown. Let cool for about 5 minutes before serving.