Prepare Lemon Filling. Cover and chill at least 8 hours.
Combine flour, baking powder, and salt; set aside.
Beat butter at medium speed with an electric mixer until creamy. Gradually add 1 3/4 cups sugar, beating at medium speed until light and fluffy. Add lemon rind, lemon juice, and vanilla; beat well. Add flour mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition.
Beat egg whites at medium speed with an electric mixer until foamy; gradually add remaining 1/4 cup sugar, beating until stiff peaks form. Fold one-third of egg whites into batter. Gently fold in remaining beaten egg whites just until blended. (Do not overmix.)
Pour batter into 3 greased and floured 9-inch round cakepans.
Bake at 350° for 18 to 22 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans, and cool completely on wire racks.
Spread 3/4 cup White Chocolate Frosting between each cake layer; carefully spread 1/2 cup Lemon Filling over White Chocolate Frosting between each cake layer. Spread remaining White Chocolate Frosting on top and sides of cake. Chill until ready to serve. Store in refrigerator.