1. Heat oven to 350 degrees F. Coat 15 x 10 x 1-inch jelly-roll pan with nonstick cooking spray. Line with waxed paper; coat paper with spray. Melt chocolate in small bowl in microwave on 100% power 1 to 2 minutes. Stir until smooth.
2. In a large bowl, beat butter, sugar and vanilla until smooth. Beat in eggs and melted chocolate. In medium-size bowl, mix flour, baking powder, baking soda and salt.
3. Alternately beat in flour mixture and buttermilk to butter mixture, ending with flour. Evenly spread in prepared pan.
4. Bake in 350 degrees F oven 25 minutes or until top springs back when pressed. Let cool in pan on rack 5 minutes. Invert onto rack; remove pan and paper. Let cool completely.
5. Slice cake crosswise into thirds, each piece 10 x 5 x 1 inches. Prepare mousse following package directions, adding cinnamon. Stack cake layers, with 3/4 cup mousse between layers and 1-1/4 cups on top. Chill 1 hour. Garnish with curls.