Heat the oven to 325°F with a rack in the middle position. Coat a 12-cup nonstick Bundt pan with cooking spray then dust with flour, tipping out any excess.
In a bowl, whisk together the flour, baking soda and salt.
In a liquid measuring cup, combine the buttermilk and lemon juice.
In a stand mixer with a whisk attachment, combine the egg whites and 3 tablespoons of sugar. Beat on medium-high until thick, billowy and very soft peaks form, about 2 minutes. Transfer the whites to a bowl and set aside, reserving the mixer bowl.
Add the remaining 2 cups of sugar and the lemon zest to the empty mixer bowl. Using the paddle attachment, mix on low until the sugar appears moistened and begins to clump, about 1 minute.
Add the 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 until incorporated after each. Scrape down the bowl and beater.
Add 1/3 of the flour mixture, then mix on low.
Add 1/2 of the buttermilk mixture then mix again. Repeat the process of adding and mixing, ending with the final 1/3 of flour.
Using a silicone spatula, gently fold the egg whites into the batter until barely combined but no white streaks remain.
Transfer the batter to the prepared pan and smooth the top.
Bake until the cake is golden brown and bounces back when gently pressed, 55-65 minutes, rotating the pan halfway through baking.
Cool the cake in the pan on a wire rack for 15 minutes, then remove from the pan and cool completely.
To serve, slice, butter and briefly toast under the broiler (use the oven’s bottom shelf).
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