Preheat oven to 350°F (175°C).

Line a 13x9-inch baking pan with foil, extending the foil over the sides.

Spray the foil-lined pan with cooking spray.

Prepare brownie batter according to package directions.

Pour the brownie batter into the prepared pan.

Bake for 15 minutes, or until the top is shiny and the center is almost set.

In a large bowl, beat cream cheese, sugar, and vanilla with an electric mixer on medium speed until well blended.

Add sour cream and mix well.

Add eggs one at a time, mixing on low speed after each addition until just blended.

Pour the cream cheese mixture over the partially baked brownie batter.

Bake for 40 minutes, or until the center is almost set.

Run a knife or metal spatula around the rim of the pan to loosen the cake.

Cool completely.

Refrigerate for at least 4 hours or overnight.

Let the cheesecake stand at room temperature for 30 minutes before serving.

Lift the cheesecake from the pan using the foil handles.

Drizzle with melted chocolate; let stand until the chocolate is firm.

Cut into 16 pieces and serve.

Store leftover cheesecake in the refrigerator.