Preheat oven to 325. Grease and line 2 9-inch cake pans with parchment.
Sift together flour and baking powder. Set aside.
Cream butter and sugar until light and fluffy.
With a mixer on slow speed, add flour mixture to the butter mixture, alternately with milk. Beat after each addition. Beat in vanilla.
Whip the egg whites until stiff but not dry. Fold them into the cake batter gently, making sure not to over mix. Pour into prepared pans.
Bake for 25 - 30 minutes or until a tester comes out clean. Place cake pans on cooling rack and allow to cool completely before removing from pans.
5 egg yolks
1 cup sugar
4 lemons, zested and juiced
1 stick butter, cut into pats and chilled
Add enough water to a medium saucepan to come about 1-inch up the side. Bring to a simmer over medium-high heat. Meanwhile, combine egg yolks and sugar in a medium size metal bowl and whisk until smooth, about 1 minute. Measure citrus juice and if needed, add enough cold water to reach 1/3 cup. Add juice and zest to egg mixture and whisk smooth. Once water reaches a simmer, reduce heat to low and place bowl on top of saucepan. (Bowl should be large enough to fit on top of saucepan without touching the water.) Whisk until thickened, approximately 8 minutes, or until mixture is light yellow and coats the back of a spoon. Remove promptly from heat and stir in butter a piece at a time, allowing each addition to melt before adding the next. Remove to a clean container and cover by laying a layer of plastic wrap directly on the surface of the curd. Refrigerate for up to 2 weeks