How to Make It
Preheat oven to 350°F. Mist 6 6-oz. ramekins with cooking spray and arrange on a large baking pan. In a medium bowl, whisk egg yolks and butter until smooth. Whisk in milk, lemon zest and juice. Whisk in sugar and flour until smooth.
In a separate medium bowl (be sure it's clean and dry), beat egg whites and salt with an electric mixer on high speed until stiff peaks form. Gently fold whites into lemon mixture. Divide batter among ramekins. Pour very hot tap water into baking pan until it reaches halfway up sides of ramekins.
Bake until cakes are puffy and light golden, about 35 minutes. Transfer ramekins to a wire rack and let cool.