In a food processor, pulse together 3/4 cup granulated sugar with lemon zest until finely ground, about 1 minute. In the bowl of an electric stand mixer fitted with the paddle attachment, whip together butter with lemon sugar mixture until pale and fluffy (scrape bowl). Mix in egg, then mix in egg whites one at a time, adding in vanilla (and lemon extract if using) with second egg white.
In the liquid measuring cup used to measure buttermilk, whisk together buttermilk and lemon juice. Working in three separate batches, beginning and ending with flour mixture, add 1/3 of the flour mixture alternating with 1/2 of the buttermilk mixture and mixing just until combined after each addition.
Divide batter among 12 paper lined muffin cups. Bake in preheated oven until toothpick inserted into center of cupcake comes out clean, about 18 - 20 minutes. Remove from oven and allow to cool several minutes in muffin tin, then transfer to a wire rack to cool slightly. Meanwhile prepare simple syrup.
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