Preheat oven to 350°F. Line muffin tins with cupcake liners or spray with nonstick cooking spray. Set aside.
To the bowl of a stand mixer fitted with the paddle attachment (or a mixing bowl - and use a hand mixer), add butter and granulated sugar. Beat on MED speed for about 2 minutes.
Scrape down the sides of the bowl (rubber spatulas are great for this), then add lemon zest, vanilla extract, lemon extract, and eggs. Beat until combined.
Sift flour and measure it into a separate mixing bowl. Add cornstarch, baking powder, baking soda and salt and whisk to combine.
Add 1/2 cup of the buttermilk, plus all of the lemon juice to the batter in the mixer, and beat until just combined.
Add half of the flour mixture, mixing until just combined. Add another 1/2 cup buttermilk, and beat in. Add remaining flour, mix, then add the final 1/2 cup buttermilk.
Adding the dry ingredients and buttermilk like this makes sure they're well incorporated without being over mixed.
Scoop the batter into the prepared muffin tins, filling them about 3/4 of the way up.
Bake for 18-20 minutes, or until a toothpick inserted in the center of a cupcake comes out with just a few moist crumbs.
Let cupcakes cool in the muffin tins for about 3-5 minutes, then remove the cupcakes to a cooling rack to finish cooling.
Once cupcakes have cooled, frost as desired.
Enjoy immediately or cover and keep at room temperature for up to 3 days.