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, melted butter and egg until a thick dough forms. Press the dough into the prepared pan in an even layer. Set aside briefly.
In the bowl of a stand mixer, beat the cream cheese until creamy and smooth, about 1 minute. Add in the eggs, one at a time, beating well after each addition. Add in the rum and vanilla extracts, the nutmeg and the entire box of powdered sugar, beating well until smooth. Lastly, beat in the eggnog and melted butter until smooth and combined.
Pour the filling mixture evenly over the bar base. Bake for approx. 37-45 minutes or until golden brown on top and mostly set. It's okay if the bars jiggle a little when moved, but they should not be sloshing around in the pan. The bars will continue to set as they cool. Remove from the oven and allow to cool completely, about 1 hour.
Refrigerate the bars to set for about 2 hours. Cut into squares and dust with additional powdered sugar before serving, if using.