• Preheat your oven to 350 degrees. In a mixing bowl, sift together all-purpose flour, cake flour, baking powder, baking soda, and salt. Whisk the dry ingredients for about 20 seconds, then set the bowl aside.
• Using a food processor, pulse 3/4 cup + 2 Tbsp of granulated sugar with lemon zest until finely ground, roughly for a minute. In the bowl of an electric stand mixer fitted with the paddle attachment, beat together the butter with the lemon sugar mixture until it becomes pale and fluffy. If your mixer's attachment doesn't scrape the bowl constantly, remember to occasionally stop and scrape down the sides and bottom of the bowl.
• Next, mix in the egg, followed by the egg whites (one at a time) and add the vanilla along with the second egg white.
• In the liquid measuring cup used to measure the buttermilk, whisk together the buttermilk and lemon juice. Add 1/3 of the flour mixture to the butter/egg mixture and mix until just combined. Then add half of the buttermilk mixture and mix until combined.
• Repeat the process once more with the flour and buttermilk mixture, and finish by mixing in the last 1/3 of the flour mixture. Don't forget to scrape down the sides and bottom of the bowl to ensure everything is well incorporated.
• Divide the batter among 12 lined muffin cups and bake in the preheated oven for approximately 18-20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean.
• Once done, let the cupcakes cool for a few minutes in the muffin tin before transferring them to a wire rack to cool further. In the meantime, prepare the simple syrup.

For the simple syrup:

• Combine 1 tablespoon of lemon juice with 1 tablespoon of granulated sugar in a small bowl. Whisk the mixture until the sugar completely dissolves. Brush this lemon-sugar mixture over the warm cupcakes, using all of it and applying about two light coats to each cupcake. Let the cupcakes cool down entirely after this step.

For the Lemon Buttercream Frosting:

• In the bowl of an electric stand mixer with the paddle attachment, beat the butter with lemon zest on medium-high speed until it becomes very pale and fluffy—almost white in color. If your paddle attachment doesn't continually scrape the bowl, remember to stop the mixer occasionally throughout the process and scrape down the sides and bottom of the bowl.
• Incorporate 1 cup of powdered sugar, then blend in the lemon juice, cream, vanilla extract, and lemon extract. Add the remaining 1 1/2 cups of powdered sugar and continue whipping the mixture until it becomes very fluffy, usually for about 4 to 5 minutes longer.
• Once the frosting is ready, either pipe or spread it over the cupcakes. For a finishing touch, you can decorate them with lemon slices, lemon drop candies, or mint leaves if desired. Store the cupcakes in an airtight container.