Step 1

Cake Layer: Preheat oven to 325°F. Spray the bottom of a 9-inch springform pan with baking spray; set aside.

Step 2

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.

Step 3

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.

Step 4

Mix in sour cream by hand. Carefully pour cream cheese mixture over cake layer.

Step 5

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.

Step 6

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.

Step 7

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.

Step 8

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.

Step 9

Spoon chocolate onto cheesecake, spreading evenly to cover top. Chill just until set.

Step 10

Store cheesecake chilled in an airtight container for up to 5 days.