How to Make It
Prepare Lemon Curd: Beat first 2 ingredients at medium speed with an electric mixer until blended. Add 4 eggs and 3 egg yolks, 1 at a time, to butter mixture, beating just until blended after each addition. Stir in lemon zest. Gradually add lemon juice to butter mixture, beating at low speed just until blended after each addition. (Mixture will look curdled.) Transfer to a large heavy saucepan.
Cook mixture over medium-low heat, whisking constantly, 14 to 16 minutes or until mixture thickens, coats the back of a spoon, and starts to mound slightly when stirred. Transfer mixture to a bowl.
Place heavy-duty plastic wrap directly on surface of warm curd (to prevent a film from forming), and cool 1 hour. Chill 8 hours.
Prepare Cake Layers: Preheat oven to 350°. Beat 1 3/4 cups granulated sugar and 1 cup softened butter at medium speed with an electric mixer until fluffy. Add 4 egg yolks, 1 at a time, beating just until blended after each addition. Stir in orange zest.
Sift together cake flour and next 2 ingredients; gradually add to butter mixture alternately with 1 cup orange juice, beginning and ending with flour mixture, beating just until blended after each addition.
Beat 4 egg whites at medium-high speed with mixer until stiff peaks form. Fold one-third of egg whites into batter; fold remaining egg whites into batter. Pour batter into 4 greased (with shortening) and floured 8-inch round shiny cake pans (about 1 3/4 cups batter in each pan).
Bake at 350° for 17 to 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans to wire racks, and cool completely (about 30 minutes).
Reserve and refrigerate 1 cup Lemon Curd. Spread remaining Lemon Curd between cake layers and on top of cake (about 1/2 cup per layer). Insert skewers 2 to 3 inches apart into cake to prevent layers from sliding. Immediately wrap cake tightly in plastic wrap, and chill 12 to 24 hours. (The layers of cake and Lemon Curd will set and firm up overnight, ripening the flavor and making the cake more secure and easier to frost.)
Prepare Frosting: Beat 1 cup powdered sugar, 1/2 cup softened butter, and 1 Tbsp. orange zest at low speed with electric mixer until blended. Add 1/2 cup reserved Lemon Curd alternately with remaining 2 cups powdered sugar, beating until blended after each addition. Increase speed to high, and beat 1 to 2 minutes or until fluffy.
Remove skewers from cake; discard skewers. Spread frosting on sides of cake. Spread remaining 1/2 cup reserved Lemon Curd over top of cake. (Adding a bit of extra Lemon Curd to the top of the cake creates a luxe decorative finish.)
Lemon Curd may be stored in an airtight container in refrigerator up to 2 weeks.