Add the sugar and creamcheese in a bowl and beat together until well blended.
Add eggs (2 at a time), each time beating until well blended.
Take a whisk (by hand) and mix in the sour cream. Mix until no white is seen.
Set aside and start preparing the chocolate.
In a saucepan, melt the butter and chocolate chips over low heat. Keep mixing until blended and all the chocolate has melted. Do not let it cook or it will burn.
Fold in the chocolate into the cheese mixture. Do not worry if it's not completely blended, a little swirly look is fine. You con't want to over mix it.
Pour over the crust in the springform pan and put in the oven.
Bake at 325°F for at least 1 1/2 hours. Cake is done if the center is firm and no longer liquidy.
Let cool at room temperature. Once cooled, place in the fridge and chill.