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.
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.
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.
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.
White Chocolate Cream Frosting
In a large mixing bowl, beat whipping cream, powdered sugar, and white chocolate liqueur or clear creme de cacao on high speed of an electric mixer until stiff peaks form (tips stand straight). Use immediately.
White Chocolate Zabaglione Sauce
In the top of a double boiler, beat 4 egg yolks with 1/4 cup white chocolate liqueur or clear creme de cacao, 1/4 cup granulated sugar, and dash salt. Place over boiling water (upper pan shouldn't touch the water). Beat on medium speed of an electric mixer until mixture nearly triples in volume and temperature of mixture reaches 145 degree F and maintains that temperature for 3-1/2 minutes (about 15 minutes total). Remove from heat. Place pan in a larger bowl of ice water and continue beating until zabaglione has cooled. In a small bowl, beat 1/2 cup whipping cream on medium speed of an electric mixer until soft peaks form (tips curl). By hand, fold about one-fourth of the whipped cream into zabaglione to lighten; fold in remaining whipped cream.