Lemon Pudding Cake
1/4 cup all-purpose flour
2 teaspoons cornstarch
1 1/4 cups sugar
5 tablespoons butter, softened
2 tablespoons grated zest and 1/2 cup juice from 4 lemons
5 large eggs, separated
1 1/4 cups whole milk, room temperature
2 quarts boiling water (I used 3 quarts since my roasting pan is extra-large)
Adjust oven rack to lowest position and heat the oven to 325 degrees. Grease an 8-inch square baking dish. Whisk the flour and conrstarch in a bowl. With the electric mixer on medium-high speed, beat 1/2 cup sugar, butter and lemon zest until light and fluffy, about 2 minutes. Beat in yolks, one at a time, until incorporated. Reduce the speed to medium-low. Add flour mixture and mix until incorporated. Slowly add milk and lemon juice, mixing until just combined.
Using a clean bowl and the whisk attachment, beat egg whites on medium high speed until soft peaks form, about 2 minutes. With mixer running, slowly add remaining sugar until whites are firm and glossy, about 1 minute. Whisk one-third of the whites into the batter, then gently fold in remaining whites, one scoop at a time, until well combined.
Place a kitchen towel in the bottom of a roasting pan and arrange prepared baking dish on the towel. Spoon batter into the prepared dish. Carefully place the pan on the oven rack and pour boiling water into the pan until water reaches halfway up sides of baking dish. Bake until the surface is golden brown and edges are set (center should still be slightly wiggly), about 60 minutes. Transfer to a wire rack and let cool about 45 minutes. Serve.