Preheat oven to 325°. Butter a 2 quart souffle dish or six 6 oz ramekins or eight 4 oz ramekins.
Choose a baking pan or baking dish for a water bath large enough to accommodate the souffle dish or the ramekins with out touching, and deep enough to hold water that will reach ¾ of the way up the sides of the molds when added.
Pour enough water into the pan to reach about halfway up the sides of the pan, and place in the oven while it is heating.
In a bowl, whisk together the sugar, flour, and salt.
In another bowl, whisk together the egg yolks, buttermilk, lemon zest and juice until blended.
Make a well in the flour mixture and pour in the egg yolk mixture; whisk the mixture together until smooth.
Place egg whites in a bowl; using a mixer fitted with the whisk attachment, beat the whites until they hold soft peaks.
Scoop 1/3 of the whites onto the yolk mixture and stir in to lighten the mixture.
Gently fold in the remaining whites, being careful not to deflate the batter.
Transfer batter to prepared dish or dishes.
Pull out oven rack holding the water bath and place the mold(s) in the bath.
Pour in more water if necessary to reach ¾ of the way up the sides of the mold(s).
Bake about 30-40 minutes for the souffle dish or 25-35 minutes for the ramekins.
Remove from the oven and serve hot, or let cool on a wire rack and serve warm or at room temperature.