Preheat the oven to 350˚ F. Lightly butter a 12-cup muffin pan and sprinkle each cup with enough sugar to cover the bottom. Beat the egg whites and cream of tartar in a large bowl with a mixer on medium-high speed until stiff peaks form, about 6 minutes.
Whisk the sugar, flour and salt in a medium bowl. In a large bowl, whisk the buttermilk, egg yolks, 1 tablespoon each lemon and lime zest, and the lemon and lime juice; whisk in the dry ingredients. Fold in the beaten egg whites with a rubber spatula until combined.
Divide the batter among the muffin cups. Set the muffin pan in a roasting pan, then place in the oven; fill the roasting pan with enough warm water to come halfway up the sides of the muffin pan. Cover the roasting pan tightly with foil. Bake until the cakes are puffed and set, about 20 minutes. Uncover and continue baking until golden, 12 to 15 more minutes. Let cool 10 minutes in the pan, then carefully use an offset spatula to pull the cakes away from the pan and unmold. Invert onto a platter. Top with the remaining 1/2 tablespoon each lemon and lime zest.