Like magic, this pudding cake separates into two layers during baking: airy and soufflélike on top, dense and custardy below. YUM! NOTE: Please make sure to read the step by step instructions for this cake.
MIX BATTER Adjust oven rack to lowest position and heat oven to 325 degrees. Grease 8-inch square baking dish. Whisk flour and cornstarch in bowl. With 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 speed to medium-low. Add flour mixture and mix until incorporated. Slowly add milk and lemon juice, mixing until just combined.
BEAT EGG WHITES Using clean bowl and 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 whites into batter, then gently fold in remaining whites, one scoop at a time, until well combined.
BAKE Place kitchen towel in bottom of roasting pan and arrange prepared baking dish on towel. Spoon batter into prepared dish. Carefully place pan on oven rack and pour boiling water into pan until water comes halfway up the sides of baking dish. Bake until surface is golden brown and edges are set (center should jiggle slightly when gently shaken), about 60 minutes. Transfer to wire rack and let cool at least 1 hour. Serve.