Heat the oven to 325℉ with a rack in its middle position. Fill a 9-X-13-inch baking dish with hot tap water to a depth of ½ inch. Place the pan in the oven while it heats. Lightly butter six 6-oz. ramekins.
Whisk the sugar, flour, and salt together in a medium bowl. In a second bowl, whisk together the egg yolks, buttermilk, lemon zest and juice until blended. Add the egg yolk mixture to the flour mixture, whisking them together until fully incorporated and smooth. The result will be a very runny batter.
Using a stand mixer with its whisk attachment or a handheld mixer or whisk, beat the egg whites in a pristinely clean bowl until they hold soft peaks. Stir ⅓ of the whites into the yolk mixture to lighten it. Gently fold in the remaining whites, taking care not to deflate the batter. Using a ladle or ice cream scoop, divide the batter evenly into the prepared ramekins.
Pull out the oven rack holding the water bath and place the ramekins in the bath, taking care not to slosh water into the puddings as you slide the rack back into place. Bake until puffed and with a wobble when gently shaken, 25 to 30 minutes. The tops may turn lightly golden, and this is fine!
Serve hot, warm or at room temperature, dusted with confectioners sugar or garnished with whipped cream or fresh berries, if you have them.
Store well wrapped puddings in the refrigerator for up to 3 days. Rewarm in a 300℉ oven for 15-20 minutes (or heat each ramekin in the microwave for 30-60 seconds) before serving.