Cake Layer: Preheat oven to 325°F. Spray the bottom of a 9-inch springform pan with baking spray; set aside.
Using an electric mixer beat together cake mix, 1 egg, milk and vegetable oil until fully combined. Pour evenly into the springform pan. Bake for 25 minutes. Set aside.
Cheesecake Layer: Meanwhile, in a large mixing bowl, use an electric mixer to beat together cream cheese, sugar, and vanilla. Beat with an electric mixer on medium speed until smooth. Add the eggs; beating on low speed just until combined. Beat in cornstarch and salt.
Mix in sour cream by hand. Carefully pour cream cheese mixture over cake layer.
Bake for 50 to 60 minutes or until at least a 2-inch area around the outside edge appears set and the center appears nearly set when gently shaken.
Cool in the pan on a wire rack for 15-20 minutes. Using a small sharp knife or icing spatula to loosen the edge of cheesecake from side of pan. Keep outer ring in place.
Cool the cheesecake completely to room temperature. Remove the outer ring. side of pan; transfer cheesecake to a serving plate. Cover and chill for 8 hours up to 24 hours.
Chocolate Topping: Before serving, in a small saucepan, bring whipping cream to a simmer. Remove from heat. Add chocolate; stir until chocolate is melted and mixture is smooth. Stir in butter. Let cool for about about 15 minutes or until the chocolate has slightly thickened, stirring once.
Spoon chocolate onto cheesecake, spreading evenly to cover top. Chill just until set.
Store cheesecake chilled in an airtight container for up to 5 days.