Coat bottoms of 2 (8- or 9-inch) round cake pans with cooking spray (do not coat sides of pan); line bottom with wax paper. Coat wax paper with cooking spray, and dust with flour; set aside.
Sift together 2 1/2 cups flour, baking powder, and salt; set aside. Combine yogurt and next 5 ingredients (yogurt through egg yolk) in a large bowl; beat at medium speed of a mixer until blended. Add 1 cup sugar, and beat well. Gradually add the milk, beating at medium-low speed 2 minutes or until sugar dissolves.
Beat egg whites (at room temperature) at high speed until foamy. Gradually add the remaining 1/2 cup sugar, beating mixture until stiff peaks begin to form. Add the flour mixture and egg white mixture alternately to the yogurt mixture, beating at low speed.
Pour the batter into prepared pan. Sharply tap pan once on counter to remove air bubbles. Bake at 375° for 10 minutes. Reduce oven temperature to 350°, and bake 28 minutes or until a wooden pick inserted in center comes out clean. Let cool in pan 10 minutes on a wire rack. Loosen cake from sides of pan, using a narrow metal spatula, and turn out onto wire rack. Peel off wax paper, and let cool completely.
Slice the cake layers in half horizontally. To slice the cake layers evenly, insert toothpicks around the outside, marking the middle. Using a serrated knife, slice the cake about 2 inches, using the toothpicks as a guide. Wrap a long piece of dental floss around edge through cut; tie floss once. Slowly tighten the floss - it will make a clean, even cut through the cake, dividing it in half. Place one layer, cut side up, on a serving plate. Spread 2/3 cup Apricot-Orange Filling over cake to within 1/2 inch of edge. Repeat procedure with remaining cake layers and filling. Frost sides and top of cake with Orange Buttercream Frosting.
Note: Don't coat the sides of the cake pans with cooking spray. The cake needs to cling to the sides as it bakes, which allows it to rise higher and gives it a smoother crust.
Note: When separating the eggs, do them one at a time into a custard cup, and pour the whites into your mixing bowl. This prevents you from having to start over if some yolk happens to get into the whites. Eggs are easier to separate right out of the refrigerator; however, for better volume, let them come to room temperature before beating.
Note: Use real vanilla and orange extracts instead of imitaion.
Note: Wrap cooled cake in heavy-duty plastic wrap, dusted with flour, and then in heavy-duty aluminum foil. Refrigerate up to 2 days or freeze up to 2 weeks. (To thaw, place the wrapped cake in refrigerator overnight.)
Note: To slice the cake in half evenly, insert toothpicks around the outside, marking the middle. Using a serrated knife, slice the cake about 2 inches, using the toothpicks as a guide. Wrap a long piece of dental floss around edge through cut; tie floss once. Slowly tighten the floss - it will make a clean, even cut through the cake, dividing it in half.
High Altitude Adjustment: Beat egg whites to soft peaks instead of stiff peaks.