Lemon slices or lemon drop candies and mint leaves for decorating(optional)
Preheat oven to 350 degrees. Sift all-purpose flour and cake flour into a mixing bowl. Add baking powder, baking soda and salt and whisk 20 seconds, set aside.
In a food processor, pulse together 3/4 cup + 2 Tbsp 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 (if you don't have the paddle attachment that constantly scrapes bowl, then occasionally throughout entire mixing process, stop mixer and scrape down sides and bottom of bowl). Mix in egg, then mix in egg whites one at a time, adding in vanilla with second egg white.
In the liquid measuring cup used to measure buttermilk, whisk together buttermilk with lemon juice. Add 1/3 of the flour mixture to the butter/egg mixture, then mix just until combined. Then add 1/2 of the buttermilk mixture to the butter/egg mixture and mix just until combined. Repeat process with flour and buttermilk mixture once more then finish by mixing in last 1/3 of the flour mixture. Scrape down sides and bottom of bowl.
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.
For the simple syrup: In a small bowl, whisk together 1 Tbsp lemon juice with 1 Tbsp granulated sugar until sugar has dissolved. Brush mixture over warm cupcakes (use all of it, about 2 light coats over each cupcake). Allow cupcakes to cool completely.
For the Lemon Buttercream Frosting: In the bowl of an electric stand mixer fitted with the paddle attachment, whip butter with lemon zest on medium-high speed until very pale and fluffy (it should be nearly white. If using a paddle attachment that doesn't constantly scrape bowl while mixing, then stop mixer occasionally throughout entire mixing process and scrape down bottom and sides of bowl). Mix in 1 cup powdered sugar, then blend in lemon juice, cream, vanilla extract and lemon extract. Add remaining 1 1/2 cups powdered sugar, then whip mixture until very fluffy, about 4 - 5 minutes longer. Pipe or spread frosting over cupcakes. Decorate with lemon slices or lemon drop candies and mint leaves if desired. Store in an airtight container.