Preheat oven to 350º (325 if high altitude). In large bowl, mix first 5 ingredients (marked with*) with electric mixer until light and fluffy. Add eggs and egg whites, 1 at a time, beating well after each addition. Combine dry ingredients in separate bowl—whisk together well. Alternate adding flour mixture and buttermilk, a little at a time. Beat well after each addition. Begin and end with flour mixture. Pour batter into 2 (9-inch) round cake pans that are coated with cooking spray. Sharply tap pans on counter a couple of times to remove air bubbles. Bake at 350º for 20 minutes. NOTE: Cakes will NOT brown. They will remain pale with slightly golden edges-sort of like a Twinkie. Cool in pans for 10 minutes on wire rack, then turn them out directly onto racks to continue cooling. Cool completely.
In bowl, mix everything EXCEPT powdered sugar in bowl and beat with mixer until light and fluffy. Make sure you use COLD cream cheese, as room-temp cream cheese makes frosting too runny. Gradually add powdered sugar and beat JUST until blended. Chill 1 hour.
Beat sugar and butter together in bowl until light and fluffy. Add eggs, one at a time, beating well after each egg. Add lemon juice. Mixture will look curdled at first—but will smooth out during cooking. In heavy pan (don’t use copper, it will react with the acid), cook over low heat until smooth, then increase to medium heat. Stir constantly—especially in corners on bottom of pan. (OR you can cook in steel bowl over simmering water). DO NOT BOIL. It’s done at 170º~ when your finger leaves clear path on back of spoon. Remove from heat. Add zest. Cover with plastic wrap (pressed onto surface of curd to prevent skin from forming) and thoroughly chill in refrigerator. It will last 1 week in fridge…2 months in freezer.
Slice both layers of COOLED cake in half, horizontally--to create 4 separate layers (I use dental floss). Place one layer section on large plate. Slather with thick layer of curd. Place remaining layers on top, one at a time, slathering with curd in between each layer--or just use frosting in between layers if you didn't make curd. Frost entire cake with frosting. May garnish with thin slices of lemon, lemon zest, mint leaves or with Lemon gel candies.