Preheat oven to 350°.
To prepare the cake, combine 3 tablespoons cocoa and cake mix in a large bowl. Prepare the cake mix with added cocoa according to package directions. Coat a 15 x 10-inch jelly roll pan with cooking spray; line bottom with wax paper. Pour batter into pan.
Bake at 350° for 25 minutes or until cake springs back when touched lightly in center. Cool the cake in pan 5 minutes. Loosen cake from sides of pan, and turn out onto a dish towel dusted with 1/2 cup powdered sugar; carefully peel off wax paper. Cool the cake 1 minute. Starting at narrow end, roll up cake and towel together. Place, seam side down, on a wire rack, and cool completely (about 15 minutes).
To prepare the filling, place baking chocolate in a glass bowl; microwave at high 2 minutes or until almost melted, stirring after 1 minute. Cool. Place the cream cheese in a medium bowl, and beat with a mixer at medium speed until smooth. Gradually add the melted chocolate and 1 cup powdered sugar, beating mixture well (mixture will be very thick). Gently stir 1/2 cup whipped topping into the filling, and gently fold in the remaining whipped topping.
Carefully unroll cake; remove towel. Spread filling over the cake, leaving a 1/2-inch border around outside edges. Re-roll cake, and place, seam side down, on a platter. Cover and chill for 30 minutes. Sprinkle with 1 teaspoon cocoa. Cut the cake into 10 slices; garnish with chocolate curls, if desired.