2-4 tsp lemon juice, depending on how strong you like the flavor
Preheat the oven to 350 degrees F. Line a standard 12-cup muffin tin with paper liners.
In a medium bowl, whisk together the flour, lemon zest, baking powder, and salt, set aside.
Using an electric mixer, beat the butter and sugar on medium-high until fluffy, 2 to 3 minutes. Beat in the eggs, one at a time, then the vanilla, scraping down the sides of the bowl as needed.
Reduce mixer speed to low and add the flour mixture in 3 additions and the milk in 2 additions, beginning and ending with the flour mixture. Mix just until combined.
Divide the batter evenly among the muffin cups, filling 1/2 full. Bake until a toothpick inserted in the center comes out clean, 15 to 20 minutes. Cool in the tin for 10 minutes, then transfer to cooling rack to cool completely before frosting.
Light Lemon Buttercream:
In a mixing bowl, combine the room temperature butter and shortening, beat using an electric mixer for 2 minutes, until light and fluffy.
Begin to add in the powdered sugar, about 1 cup at a time. Slowly beat the mixture until the powered sugar is incorporated. Continue this process, adding enough powdered sugar to make sure the frosting isn’t too wet.
Add in the vanilla and lemon juice, beat until just combined.
Transfer a portion of the frosting to a large ziploc bag or piping bag. Push or squeeze the frosting down, getting rid of any air pockets. If using a ziploc bag, cut half one corner, making a hole between the size of a dime and a penny.
Begin to pipe the frosting onto a cupcake, starting with the outside edge and working your way into the center of the cupcake, progressively stacking the frosting as you go around in circles.