Preheat the oven to 350°F (180°C).
In a large bowl, whisk together 2/3 cup of the granulated sugar and the flour. Add the lemon juice, zest, and buttermilk and whisk until smooth.
In a separate bowl, whisk together the butter and the egg yolks. Stir the egg yolk mixture into the buttermilk mixture.
Using the whisk attachment of a mixer, whip the egg whites until frothy. Sprinkle in the remaining 1/4 cup granulated sugar and whip until soft peaks form, about 1 minute. Fold the egg whites into the egg yolk and buttermilk mixture. The batter should be smooth and thick.
Pour the batter into an 8-inch square baking dish (or see note above if you prefer individual ramekins) and place the dish in a water bath with the water halfway up the sides of the dish. Bake 23 to 30 minutes, until slightly brown and beginning to crack but still jiggly. The smaller the baking dish, the less time will be needed to reach this stage. Cool to room temperature.