For Vanilla Buttercream Frosting (for Lemon, see the section above labeled "With Lemon Buttercream Frosting"):
How to Make Lemon Cupcakes from Scratch:
Preheat the oven to 350°F.
Line a 12-cup muffin pan with cupcake liners.
In a medium bowl, cream butter and sugar together until pale and creamy.
Add the egg whites, vanilla extract, and lemon extract. Mix until combined.
For best results, whisk the flour, baking powder, and salt together in another bowl.*
Pour half of the dry ingredients into the wet ingredients. Mix some. Then add the other half of the flour mixture.
Add the milk, lemon juice, and lemon zest. Mix until just combined. Do not overmix the batter.
Spoon batter evenly into the cupcake liners, filling them about 2/3 full. I like to use an ice cream scoop to do this.
Bake for 18-20 minutes or until done (an inserted toothpick comes out clean).
Remove from oven and allow to cool completely before frosting.
How to Make Vanilla Buttercream Frosting:
In a large mixing bowl, beat the softened butter until smooth.
Add the powdered sugar, one cup (or less) at a time, on low speed.
Add the milk, vanilla extract, and salt. Increase to medium speed. Stop to scrape down the sides, with a rubber spatula, to the bottom of the bowl as needed.
Beat at medium-high speed until smooth and fluffy.
With the rubber spatula, press the frosting against the sides of the bowl to eliminate any air bubbles.
Spread or pipe on completely cooled cupcakes. Or refrigerate until ready to use.**
For Lemon Buttercream Frosting, please see the section above labeled “With Lemon Buttercream” for adjustments.* I prefer to use one bowl, so I just add the flour, baking powder, and salt on top of my wet ingredients. I do lightly toss them together with my fingers before mixing them fully into the wet ingredients though.**If you refrigerate the buttercream, please bring it to room temperature then whip again, with an electric mixer, before using. This will bring back the fluffy, creamy texture.