Heat the oven to 300°F (150°C). Butter and lightly sugar six 4-ounce ramekins, tapping out any excess sugar.
In a stand mixer on medium to medium-high speed, beat the egg whites until they hold soft peaks, about 5 minutes. In a separate bowl, sift the sugar with the flour and salt.
In a stand mixer using the whisk attachment and a clean bowl, combine the buttermilk, lemon juice, egg yolks, and lemon zest. Gradually add the flour mixture, then switch to a spatula and gently fold in the egg whites. Divide the batter among the prepared ramekins.
Place the puddings in a water bath (that is to say, place the ramekins in a large roasting pan, place the pan on the oven rack, and then carefully fill the pan with enough hot water to come halfway up the sides of the ramekins). Cover the entire water bath situation with a large sheet of aluminum foil, crimping it to seal it tightly.
Bake until the puddings puff and the surface seems almost firm, 25 to 30 minutes. Then uncover the ramekins and continue baking until the tops are lightly golden and the puddings spring back when touched, about 15 minutes more.
Transfer the ramekins to a wire rack to cool for a few minutes. Run a knife around the edge of the ramekins and then invert each onto a plate.
Invert the puddings onto individual serving plates and serve warm.