Preheat oven to 325°F.

Butter two 6-ounce ramekins. Put about 1 tablespoon of sugar in one, press the open ends of the ramekins together and shake to coat. Discard excess sugar.

In a medium bowl, whisk egg white and salt until you have stiff but glossy peaks.

In another bowl, whisk together egg yolk, buttermilk, lemon zest, lemon juice, and sugar until smooth and uniform. Whisk in flour until just mixed.

Fold in egg whites and pour into ramekins.

Place ramekins in an 8x8 inch baking pan, place in oven, and add boiling water to a depth of 1/2 inch.

Bake for about 40 minutes, or until tops are golden and spring back to the touch.

Remove from water bath, run a sharp knife around the edges, and invert onto plates.