The solution: We eventually deduced that it was the acidity of the citrus juices that made the difference. Because the juice lightly clabbered the milk-based batter, causing it to thicken, the frothy upper layer became stiffer and more stable and acidic juice undercut the thickening power of the flour, making it a more tender custard. To shore up the cake part of those variations made with coffee, chocolate, and vanilla, we tried adding an extra egg white. We liked the results so much that we ended up using the extra white in all of the recipes. The excessive thickness of the pudding layer in the nonacidic variations was easily fixed by reducing the amount of flour.

The following pudding cakes can be made in any of the following: six 3/4 cup custard cups; four 1 1/3 cup ramekins or miniature souffle cups; one 9-inch round cake pan; or one 8-inch round cake pan. All pudding cakes, regardless of pan size, require the same baking time.

LEMON PUDDING CAKE

Serves 4 to 6

2 tablespoons unsalted butter, softened, plus enough to grease the baking pan
1/2 cup plus 2 tablespoons sugar
1/8 teaspoon salt
3 large egg yolks
3 tablespoons all-purpose flour
2-3 teaspoons grated zest and 1/4 cup strained juice from 1 or 2 lemons
1 cup milk
4 large egg whites, at room temperature

1. Adjust oven rack to center position and heat oven to 325 degrees. Lightly butter pan or baking molds of choice. Lay folded dish towel in bottom of roasting pan and set molds or pan inside. Bring several quarts of water to boil for water bath.

2. Meanwhile, in mixing bowl mash 2 tablespoons butter together with sugar and salt with back of wooden spoon until crumbly. Beat in yolks, then flour, mixing until smooth. Slowly beat in lemon zest and juice, then stir in milk. Beat egg whites to stiff, moist peaks. Gently whisk whites into batter just until no large lumps remain.

3. Immediately ladle (don’t pour) batter into pan, custard cups, or ramekins. Set baking pan on oven rack. Pour enough boiling roasting pan to come halfway up sides of baking pan or molds. Bake until pudding cake center is set and springs back when gently touched, about 25 minutes. Remove roasting pan from oven and let pan or molds continue to stand in water bath for 10 minutes. Pudding cakes can be served warm, at room temperature, or even chilled.

ORANGE PUDDING CAKE

Serves 4 to 6

Follow the recipe for Lemon Pudding Cake, making the followong changes: Substitute the juice (1/4 cup) and zest from 1 medium navel orange for the lemon juice and zest. Add 2 tablespoons lemon juice along with the orange juice.

COFFEE PUDDING CAKE

Serves 4 to 6

This cake tastes best when made with super-strong coffee. The easiest way to make the strong coffee needed for this recipe is to cover one-third cup finely ground coffee with two-thirds cup boiling water and let it stand for five minutes, then drip through a coffee filter.

Follow the recipe for Lemon Pudding Cake, making the following changes: For lemon juice and zest, substitute 1/3 cup cooled strong coffee and 2 tablespoons coffee-flavored liqueur. Decrease sugar from 1/2 cup plus 2 tablespoons to 1/2 cup, and decrease flour from 3 to 2 tablespoons.

CHOCOLATE PUDDING CAKE

Serves 4 to 6

Follow the recipe to Lemon Pudding Cake, making the following changes: Make a rather thick cocoa paste by slowly stirring 1/2 cup boiling into 1/3 cup unsweetened cocoa. Cool the paste slightly, then stir in 1 tablespoon dark rum. Substitute cocoa paste for lemon juice and zest. Decrease flour from 3 to 2 tablespoons.

VANILLA-BOURBON PUDDING CAKE

Serves 6

This delicious version is rich enough to serve six amply. If possible, bake it in cups rather than a pan, and eat it warm.

Follow the recipe for Lemon Pudding Cake, making the following changes: Decrease flour from 3 to 2 tablespoons, substitute I tablespoon pure vanilla extract and 1 tablespoon bourbon whisky for lemon zest and juice, and increase milk from 1 to 1 1/3 cups. Serve with Bourbon Butter Sauce.

BOURBON BUTTER SAUCE

8 tablespoons unsalted butter
2/3 cup sugar
2 tablespoons bourbon
1/2 teaspoon grated nutmeg
1/8 teaspoon salt
1 large egg, beaten until foamy

Heat first five ingredients plus 2 tablespoons water in small, heavy-bottomed saucepan over medium-low heat, stirring occasionally, until bubbly around the edges. Remove from heat. Beat egg into hot butter mixture. Return to burner, bring to boil over medium-low heat, stirring constantly. Cook until thickened, about 1 minute. Spoon sauce over each pudding cake. Pass leftover sauce separately.