2 1/4 cups cake flour
1 tablespoon baking powder
1/2 teaspoon salt
1 1/4 cups whole milk or buttermilk, at room temperature
4 large egg whites, at room temperature
1 1/2 cups sugar
2 teaspoons grated lemon zest
1 stick (8 tablespoons) unsalted butter, at room temperature
1/2 teaspoon pure lemon extract
Getting Ready: Preheat oven to 350 degrees F. Butter two 9-x-2-inch round cake pans and line the bottom of each pan with a round of buttered parchment or wax paper. Put the pans on a baking sheet.  If you are making cupcakes, line cupcake wells with paper liners.

To Make The Cake: Sift together the flour, baking powder and salt.
Whisk together the milk and egg whites in a medium bowl.
Put the sugar and lemon zest in a mixer bowl or another large bowl and rub them together with your fingers until the sugar is moist and fragrant. Add the butter and, working with the paddle or whisk attachment, or with a hand mixer, beat at medium speed for a full 3 minutes, until the butter and sugar are very light. Beat in the extract, then add one third of the flour mixture, still beating on medium speed. Beat in half of the milk-egg mixture, then beat in half of the remaining dry ingredients until incorporated. Add the rest of the milk and eggs, beating until the batter is well blended, then add the last of the dry ingredients.
Finally, give the batter a good 2-minute beating to ensure that it is thoroughly mixed and well aerated. Divide the batter between the two pans and smooth the tops with a rubber spatula.

Bake for 30 to 35 minutes, or until the cakes are well risen and springy to the touch – a toothpick inserted into the centers should come out clean.  (Cupcakes will need to bake about 20-22 minutes.) Transfer the cakes to cooling racks and cool for about 5 minutes, then remove them from the pans. Cool completely before frosting.

For the frosting:
This is my own simple buttercream starting point.  I start with this, and then I add more/less cream or powdered sugar depending on what consistency I want the frosting to be.  There is a lovely buttercream tutorial at My Baking Addiction if you would like more information.

1 cup butter (2 sticks), very soft
2-3 oz. cream cheese (you can leave this out, I just like to add it because I cuts the sweetness somewhat)
1 tsp vanilla
4 cups powdered sugar
4-8 Tbsp cream or milk

Cream the butter and cream cheese, add the vanilla and powdered sugar 1 cup at a time and blend well.   Add enough cream or milk to bring the frosting to a spreadable consistency.  And then, whip it, and whip it good.