2 large eggs, separated, at room temperature
2 tablespoons unsalted butter, softened
2/3 cup sugar
2 tablespoons all-purpose flour
2 tablespoons lemon zest
1/8 teaspoon salt
¼ cup fresh lemon juice from 2 lemons
1 cup milk, at room temperature
Lemon Zest or Mint garnish
1. Adjust an oven rack to the middle position and heat the oven to 350 degrees. Butter six 6-ounce ramekins (or spray with Pam).
2. With an electric mixer on medium-high speed, beat the egg whites to soft peaks. Transfer to a bowl and set aside. Beat the butter and sugar together on medium-high speed until fluffy, about 2 minutes, scraping down the bowl as necessary. Reduce the speed to medium and add the egg yolks, mixing until incorporated. Add the flour, lemon zest, and salt and beat until combined. Add the lemon juice and milk and beat until incorporated. Using a rubber spatula, fold in the egg whites. Pour the batter into the prepared ramekins. Place the dishes in a large baking pan and add enough boiling water to the pan so that it reaches halfway up the sides of the ramekins.
3. Bake until the tops are golden and the center springs back when gently pressed, 25 to 35 minutes. Transfer the ramekins to a rack to cool completely, at least 1 1/2 hours. (The pudding cakes can be refrigerated for up to 2 days. Allow to sit at room temperature for 30 minutes before serving.) To serve, run a paring knife around the edges of the dish and invert onto a plate.