Preheat oven to 350 degrees F. Grease and flour two 9x1 1/2-inch cake pans. Combine cake mix, VOSKOS(R) Nonfat Vanilla Greek Yogurt, water, vegetable oil, and eggs in a large mixing bowl. Beat with an electric mixer on medium speed until smooth. Scrape sides of bowl; fold in grated chocolate.
Divide batter between prepared pans. Bake for 25 to 30 minutes or until a wooden toothpick inserted near centers comes out clean. Cool cakes in pans on wire racks 10 minutes. Remove from pans; cool cakes on wire racks. Fill and frost layers with Creamy Frosting (see below).
Creamy Frosting: Combine butter, VOSKOS(R) Nonfat Vanilla Greek Yogurt, and vanilla in large mixing bowl. Beat with an electric mixer for 30 seconds. (Mixture will appear curdled.) Gradually beat in powdered sugar until frosting is spreadable.