Step 1

Place lemon juice in small bowl or custard cup. Sprinkle gelatin over. Let stand 15 minutes.

Step 2

Whisk sugar, orange juice, yolks, orange peel, and lemon peel in heavy large saucepan to blend. Add butter. Whisk over medium heat until curd thickens and bubbles begin to appear at edges, about 9 minutes. Remove from heat. Add gelatin mixture. Whisk until gelatin dissolves. Transfer curd to small bowl. Press plastic wrap onto surface of curd. Chill overnight.

Step 3

Preheat oven to 350°F. Butter and flour three 9-inch-diameter cake pans with 1 1/2-inch-high sides. Sift flour, 1 1/4 cups sugar, baking powder, and salt into medium bowl. Whisk oil and egg yolks in large bowl until well blended. Whisk in sour cream, then milk, orange peel, and vanilla. Whisk in dry ingredients in 3 additions. Using electric mixer, beat egg whites in another large bowl until soft peaks form. Gradually add remaining 2/3 cup sugar, beating until whites are stiff but not dry; fold into batter in 4 additions. Divide batter among prepared pans.

Step 4

Bake cakes until tester inserted into center comes out clean, about 20 minutes. Cool cakes in pans 5 minutes. Cut around pan sides. Turn cakes out onto racks and cool completely. Place 1 cake layer, flat side up, on 8-inch tart pan bottom or platter. Spread 1 cup curd over. Top with second cake layer, flat side down. Spread 1 cup curd over. Top with third cake layer, flat side down. Cover; refrigerate assembled cake.

Step 5

Place 1 cake layer, flat side up, on 8-inch tart pan bottom or platter. Spread 1 cup curd over. Top with second cake layer, flat side down. Spread 1 cup curd over. Top with third cake layer, flat side down. Cover; refrigerate assembled cake.

Step 6

Using electric mixer, beat cream cheese and butter in medium bowl until smooth. Beat in powdered sugar, then 3/4 cup orange curd. Spread frosting over cake. DO AHEAD Can be made 1 day ahead. Cover with cake dome; refrigerate.