Position rack in the center of oven, preheat 350. Butter 8-6oz ovenproof ramekins or custard cups; arrange in baking dish or roasting pan.
In large bowl, whisk butter, 2/3 cup sugar, egg yolks until smooth and light; about 1 minute. Add flour, salt, enough milk to whisk the flour smoothly into egg mixture. Then whisk in remaining milk, lemon juice until smooth. The mixture will be runny.
Put egg whites in a large bowl. Beat on medium speed until they begin to foam, 30-60 seconds. Increase speed to high and beat just until they hold soft peaks, another 1-2 minutes. Reduce speed to medium. With mixer running, very slowly sprinkle in remaining sugar; about a minute. Stop mixture and scrape bowl. Beat on high until white hold medium-firm peaks, about 30 seconds.
Scrape 1/3 egg whites onto yolk mixture, sprinkle zest on top, whisk until combined. Gently incorporate the remaining whites into the batter, using the whisk in a folding/stirring motion. The batter will still be thin.
Portion the mixture evenly among the ramekins; the cakes don't rise much, so you can fill within 1/8" of the top. Pull out the oven rack and put the baking dish full of ramekins on the rack. Pour warm water into baking dish 1/2 way up the ramekins.
Bake until tops are light golden and when touched with finger, they should be spongy and spring back, but hold a shallow indentation, 20-25 minutes.
Using tongs, carefully transfer ramekins to a wire rack. Let cool to room temperature then refrigerate at least 2 hours, up to 24 before serving. Top with whipped cream, if desired.