1. Heat oven to 350 degrees F. Lightly coat bottom of a 10-inch angel food cake pan with nonstick cooking spray. Whisk together 3/4 cup of the sugar, the flour, cocoa powder, baking soda and salt. Stir in chocolate.
2. Whisk egg whites and cream of tartar on high speed 2 to 3 minutes or until soft peaks form. Add remaining 3/4 cup sugar to bowl and beat for 3 minutes or until stiff, glossy peaks form. Beat in vanilla.
3. Gently fold flour mixture into egg whites until fully combined. Pour batter into prepared pan and smooth top. Gently tap on counter to remove any air bubbles. Bake at 350 degrees F. for 35 to 40 minutes or until toothpick inserted in center tests clean. Remove from oven; invert over wire rack until completely cool. Run a thin knife around edges and center to release cake from pan, if necessary.Chocolate Sauce.
4. Place chocolate in small bowl. Heat cream in saucepan over medium heat until just simmering. Pour over chocolate. Let stand 4 minutes to melt. Stir until smooth. Serve with cake.