Heat the oven to 325ºF with a rack in the middle position. Rub 1 tablespoon of the butter evenly over the pan, then use a pastry brush to ensure it gets into all corners. Sprinkle in a bit of sugar, then turn the pan to evenly coats all surfaces.

In a bowl, whisk together the flour, baking soda and salt. In a liquid measuring cup, combine the buttermilk and lemon juice; set aside.

In a stand mixer with a whisk attachment, whip the egg whites on medium-high until light and foamy, about 1 minute. With the mixer running, slowly sprinkle in 3 tablespoons of the sugar and continue to whip until the whites are thick and glossy and hold soft peaks, about 1 minute.

Transfer the whites to a bowl and set aside, then add the remaining 2 cups of sugar and the lemon zest to the stand mixer's bowl.

Using the paddle attachment, mix the sugar and zest on low until the sugar appears moistened and begins to clump, about 1 minute.

Add the remaining 12 tablespoons of butter and mix on medium-low until the mixture is cohesive, then increase the mixer to medium-high and beat until pale and fluffy, about 3 minutes. Reduce the mixer to low and add the yolks, one at a time, mixing after each.

Add a third of the flour mixture, then mix on low. Add half of the buttermilk mixture, then mix again. Repeat the process of adding and mixing, ending with the final third of flour.

Use a silicone spatula to fold ⅓ of the whipped egg whites into the batter until combined, then gently fold in the remaining whites until barely combined. Transfer the batter to the prepared pan and smooth the top.

Bake until golden brown and bounces back when gently pressed, 50 to 60 minutes, rotating the pan halfway through baking. Cool the cake in the pan on a wire rack for 10 minutes, then remove from the pan and cool completely.