Instead of using icing, frost this lemon-filled cake mix fix-up with a soft meringue and bake in a hot oven until the swirls of meringue are golden.
Heat oven to 350 degrees F. Coat two 8 x 2-inch round layer-cake pans with cooking spray. Line bottoms with waxed paper; coat paper with spray.
Place cake mix, yogurt, buttermilk, oil, eggs and rind in large bowl. On low speed, beat until combined. Increase speed to medium; beat 2 minutes. Divide between pans.
Bake in 350 degree F oven 35 minutes, until toothpick inserted in centers tests clean. Let cakes cool in pans on wire rack 10 minutes. Turn cakes out onto rack and let cool completely. Increase oven temperature to 450°F.
Cut each cake in half horizontally with a serrated knife, using toothpicks as a guide.
Place one cake layer on baking sheet. Spread a third of lemon curd (about 1/3 cup) evenly over top. Place another layer on top of curd; spread with curd. Repeat layering 2 more times with cakes and curd, leaving top cake layer plain.
Beat powdered egg whites and warm water in clean large bowl until powdered whites are dissolved, 2 minutes. Beat on medium-high until foamy. Add cream of tartar; beat, adding sugar, 1 tablespoon at a time, until stiff, glossy peaks form. Frost cake with meringue.
Bake cake in 450 degree F oven 5 to 7 minutes, until meringue is nicely browned. Cool 10 minutes. Place in refrigerator; cool completely. Remove to platter. Cut with serrated knife.