Reserve 1/4 of cake mix for filling
Wrap the outside and bottom of 10 inch springform pan. Spray the inside with cooking spray or baking spray. Mix dry cake mix with soft butter with an electric mixer on low speed. Press into the bottom and 1 1/2 inches up sides of pan.
In a large bowl beat reserved cake mix with cream cheese, chocolate, sour cream, and sugar. Add eggs one at a time until just blended. Pour into crust. Drop food coloring into the center of filling and swirl 2-3 times with the tip of a sharp knife. Bake cheesecake for 65-75 minutes, until cake is set at least 2 inches from edges, but still a little jiggly in the center. Turn oven off. Leave oven door open 4 inches. Leave cheesecake in oven for 30 minutes. Remove and place on cooling rack for 30 minutes. Chill cheesecake for at least 6 hours or overnight before garnishing and serving.
To serve, run a small metal spatula around the outside of cake and then release from pan. Garnish with chocolate chips, whipped cream or chopped chocolate truffles. Cut into small slices to serve as it's very rich. Serves 12-14. Store covered in refrigerator or freeze.