Preheat the oven to 350°F. Liberally spray a 13 x 9-inch non stick metal baking pan with cooking spray. Set aside.
In the bowl of a stand mixer, beat together the cake mix, ½ cup melted butter and 1 large egg. Beat until fully moistened. Press into the bottom of the pan.
In the same mixing bowl whip together the softened cream cheese, powdered sugar and melted butter until creamy
Add the eggs one at a time beating well after each addition. Stop and scrape the sides of the bowl as needed.
Add the melted chocolate to the creamed mixture. Beat for one minute or until fully combined.
Pour the filling over the crust. Bake for 40-50 minutes until the center still moves when gently shaken.
Cool on a baking rack in the pan. Store chilled.
Serve chilled or bring to room temperature prior to serving topped with a dollop of whipped cream, if desired.