Preheat oven to 350°F. Spray a 9"x13" baking pan with nonstick cooking spray, and set aside.
In a large bowl, combine cake mix, one egg, and ½ cup of melted butter with a hand mixer on medium speed. Pat into the bottom of the prepared pan, covering the entire bottom completely.
Add cream cheese to large bowl and beat with mixer until smooth. Add in the remaining eggs and vanilla extract, and mix well. Slowly add in the remaining butter and powdered sugar, combining until smooth. Spread evenly over cake mixture.
Bake for 40-45 minutes or until center has set slightly - do not overcook. Cool cake completely, then cut into small squares and serve.