Preheat oven to 350 degrees F.

Butter six (6 ounce) custard cups with 1 1/2 teaspoons butter, then place in a large roasting pan.

In a large bowl, cream 1 1/2 tablespoons butter, the sugar and lime peel using an electric mixer. Add the egg yolks 1 at a time, beating well after the addition of each. Beat in the flour, then the lime juice. Gradually mix in milk (mixture will be thin and may look curdled).

In a large bowl beat the egg whites until stiff. Gently fold the meringue into the yolk mixture in 3 additions. (The mixture will be runny; do not overmix.) Divide the pudding-cake mixture among the prepared custard cups. Pour enough hot water into the roasting pan to come halfway up the sides of dishes. Bake until puffed and lightly firm to touch, about 30 to 40 minutes. Sprinkle the Custard Cakes with powdered sugar and serve warm, garnished with fresh mint sprigs if desired.