Preheat oven to 350 degrees F. Line a 13x9" baking pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil liberally with cooking spray; set aside.
In a large bowl, combine the cake mix, one (1) stick of melted butter and one (1) egg, stirring with a rubber spatula until combined and thick. Press the mixture into the prepared pan in an even layer. Sprinkle the layer with the Reese's Minis.
Wipe out the bowl and add in the cream cheese and Reese's chocolate spread. Using an electric mixer, beat for about 30 seconds - 1 minute or until combined. Add in the eggs, one at a time, beating well after each addition. Add in the remaining stick of melted butter, beating well. Lastly, add in the powdered sugar all at once, beating until incorporated.
Pour the mixture over the crust layer. The pan will weigh about one ton. Pop the pan in the oven and bake for approx. 35-40 minutes or until the center is just about set and only slightly jiggly. It's okay if it jiggles a little bit; do NOT over bake! Allow the bars to cool completely, then refrigerate for about 2 hours or until chilled before cutting into squares.
Dust the tops with powdered sugar, if desired. Store leftovers in the fridge.