Preheat oven to 350°F and spray a 9x13 baking dish.
Mix cake mix, butter, eggnog, and rum extract until a soft cookie dough forms.
Press the cookie layer evenly into the prepared pan.
Beat softened cream cheese and sugar until smooth.
Add eggs, eggnog, rum extract, and nutmeg. Mix until creamy.
Pour cheesecake mixture over the cookie layer and spread evenly.
Bake until the center is set and edges are lightly golden.
Cool completely, then refrigerate until firm.
Slice into bars and serve chilled.
Notes
Use full-fat block cream cheese for the smoothest filling. Let the bars chill completely before slicing so the cheesecake layer sets. Rum extract is optional but gives the strongest eggnog flavor. Store refrigerated and cut with a clean knife for neat bars.