Step 1

PreparationMake lemon curd: Combine 2 1/3 cups sugar and 2 teaspoons cornstarch in heavy medium saucepan. Gradually whisk in fresh lemon juice. Whisk in eggs and yolks; add butter. Whisk over medium heat until curd thickens and boils, about 12 minutes. Pour into medium bowl. Refrigerate until cold, at least 5 hours. (Can be prepared 1 week ahead. Cover and keep refrigerated.)

Step 2

Make frosting: Beat powdered sugar and 1 1/4 cups lemon curd in large bowl just until blended. Beat cream in medium bowl until firm peaks form. Fold cream into curd mixture in 3 additions. Chill until firm, at least 4 hours.

Step 3

Make cake: Preheat oven to 350°F. Butter and flour three 9-inch-diameter cake pans with 1 1/2-inch-high sides; line bottoms with parchment paper. Whisk 1 1/2 cups cake flour, 1/2 cup sugar, 2 1/2 teaspoons baking powder and ‟ teaspoon salt in large bowl. Add 4 yolks, 1/4 cup vegetable oil, orange juice, lemon peel and 3/4 cup curd to bowl (do not stir). Combine whites and 1/4 teaspoon cream of tartar in another large bowl. Using electric mixer, beat whites until soft peaks form. Gradually add remaining 1 cup sugar, beating until stiff but not dry. Using same beaters, beat yolk mixture until smooth. Fold whites into yolk mixture in 3 additions.

Step 4

Divide batter equally among prepared pans. Bake cakes until tester inserted into center comes out clean, about 25 minutes. Cool cakes in pans on racks 15 minutes. Turn cakes out onto racks; peel off parchment. Cool cakes completely.

Step 5

Spoon 1 cup frosting into pastry bag fitted with plain round tip; refrigerate bag. Place 1 cake layer on cake platter. Spread top of cake layer with 1/3 cup curd, then 1 cup frosting. Top with second cake layer; spread with 1/3 cup curd and 1 cup frosting. Top with third cake layer. Spread remaining frosting over top and sides of cake. Spread remaining curd over top of cake, leaving 3/4-inch plain border around edge. Pipe chilled 1 cup frosting in bag in small mounds around edge of cake. (Cake can be prepared 1 day ahead; refrigerate.) Place lemon slices between mounds of frosting. Slice cake and serve.