Preheat the oven to 325˚ F. Lightly grease the inside of six to eight 8-ounce ramekins with the softened butter and coat with sugar. Prepare a baking dish or roasting pan by lining the inside of the pan with a clean kitchen towel. Place the prepared ramekins on top of the towel inside the baking dish.
In a medium bowl, combine the flour, ¾ cup of the sugar and salt. Add in the lemon zest and rub the zest into the flour mixture until evenly combined and fragrant. In a large bowl, combine the buttermilk, lemon juice and egg yolks. Whisk to blend, and set aside.
In a separate medium bowl, combine the egg whites and the remaining 1 tablespoon of sugar. Whisk or beat until soft peaks form, being careful not to over beat. Add the flour mixture to the bowl with the buttermilk and whisk until well incorporated. Add 1/3 of the whipped egg whites to the bowl and fold in gently with a rubber spatula. Repeat with the remaining whites in two more additions.
Once the batter is evenly combined and all the egg whites have been incorporated, divide evenly between the prepared ramekins. Fill the baking dish or roasting pan with hot water so that it reaches about halfway up the sides of the ramekins.
Tent the pan with foil, transfer to the oven and bake for 20 minutes. Remove the foil and bake for 20-30 minutes more, just until the cakes rise up and are lightly browned on top.
Transfer the ramekins to a cooling rack and let cool to room temperature. (If you have home canning equipment, I found a jar gripper very useful for this!) When ready to serve, gently run a paring knife around the edges of each cake and invert onto serving plates. Top as desired with candied lemon slices, whipped cream, etc.