After the cake layers are drenched with fresh orange syrup, they become moist and delicate. Transfer them carefully to a serving plate for frosting.
Preheat oven to 350°.
Coat 2 (9-inch) round cake pans with cooking spray, and line bottoms of the pans with wax paper.
Lightly spoon sifted cake flour into dry measuring cups; level with a knife. Combine sifted flour, baking soda, and salt, stirring with a whisk. Place butter in a large bowl, and beat with a mixer at medium speed until light and fluffy (about 2 minutes). Gradually add 1 3/4 cups sugar, 1 tablespoon at a time, beating until well blended. Beat in orange rind and vanilla. Add egg whites, 1 at a time, beating well after each addition.
Combine buttermilk and milk. Add flour mixture and buttermilk mixture alternately to butter mixture, beginning and ending with flour mixture. Pour batter into prepared pans; sharply tap pans once on counter to remove air bubbles. Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 20 minutes on a wire rack; remove from pans. Cool completely on wire rack.
Combine juice and 1/4 cup sugar; stir until sugar dissolves. Pierce the cake layers liberally with a wooden pick. Slowly drizzle juice mixture over cake layers.
Carefully place 1 layer on a plate; spread with 1/3 cup marmalade. Top with remaining layer; spread remaining marmalade on top of cake. Fold sour cream into whipped topping; spread over sides of cake. Cover and chill at least 2 hours.