Be sure to allow enough time for the filling to chill before preparing the cake.
Grease 3 (9-inch) round cake pans. Line bottoms with wax paper; grease wax paper, and dust with flour. Tap out excess flour; set pans aside.
Beat butter at medium speed with an electric mixer until creamy; gradually add 1 1/2 cups sugar, beating well. Stir in orange extract and rind.
Combine flour and next 3 ingredients in a separate bowl; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at low speed until blended after each addition.
Beat egg whites in a separate bowl at high speed with an electric mixer until foamy. Add remaining 1/4 cup sugar, 1 tablespoon at a time, beating 2 to 4 minutes or until stiff peaks form and sugar dissolves. Fold one-third of egg white mixture into batter. Gently fold in remaining egg white mixture into batter. Divide batter evenly among prepared pans. Bake at 350° for 20 to 25 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.
Place 1 cake layer on a cake plate, and brush with one-third of Orange Syrup; top with 1/2 cup of Orange Filling. Spoon half of chopped mango over filling. Repeat procedure with next layer. Top with last cake layer, and brush with remaining one-third of syrup. Spread Orange Buttercream Frosting over top and sides of cake. Spoon remaining filling on the top center of cake. (We let the filling run down the sides for a casual look.) Garnish, if desired. Serve immediately, or chill before serving.