1. Grease a 15x10x1-inch baking pan. Line bottom with parchment paper or waxed paper; grease paper. Set aside. In a small saucepan combine butter and bittersweet chocolate. Heat and stir over low heat until melted; set aside to cool. Allow eggs to stand at room temperature for 30 minutes.
2. Preheat oven to 350 degree F. In a large mixing bowl, beat eggs slightly. Add sugar, vanilla, and cinnamon; beat with an electric mixer on high speed for 10 minutes. Sift about one-third of the flour over egg mixture. Gently fold in flour. Repeat sifting and folding in one-third of the flour at a time. Gently fold in melted chocolate mixture. Spread batter in prepared pan.
3. Bake in the preheated oven for 18 to 20 minutes or until a wooden toothpick inserted near the center comes out clean. Cool cake in pan on a wire rack for 10 minutes. Remove cake from pan and peel off paper. Cool cake completely on a wire rack. Cut cake into 24 squares or 2- to 2-1/4-inch circles.
4. Just before serving, place a cake square or circle on each of 12 dessert plates. On each cake square or circle, spread about 3 tablespoons White Chocolate Cream Frosting. Top each with a second cake square or circle. Spoon White Chocolate Zabaglione Sauce over cakes, allowing some to flow down sides. If desired, garnish with fresh berries and chocolate curls. Makes 12 servings.