Preheat oven to 350 degrees F. Line a 13x9x2-inch baking pan with foil, extending foil over the pan edges. Lightly grease foil. Set pan aside. For crust, in a large bowl, stir together cookie mix and flour. Using a pastry blender, cut in butter until mixture resembles fine crumbs. Stir in 1/2 chopped macadamia nuts. Press crumb mixture into prepared pan. Bake in the preheated oven for 12 to 15 minutes or until edges are lightly browned.
For filling, in a medium bowl, whisk together egg yolks, sweetened condensed milk, eggnog, rum, vanilla, and 1/4 teaspoon nutmeg until well combined. Carefully pour the filling over the hot crust.
Bake for 15 to 20 minutes more or until set. Cool in pan on a wire rack for 1 hour. Cover and chill in the refrigerator for 1 hour or until completely cool.
Using the foil, lift the uncut bars out of the pan; pull foil from bar edges. Cut into bars.
If desired, in a medium bowl, combine whipping cream and powdered sugar. Beat with an electric mixer on medium speed until soft peaks form (tips curl). If desired, serve bars topped with whipped cream mixture sprinkled with chopped macadamia nuts and additional nutmeg. Makes 36 bars.
Layer bars between sheets of waxed paper in an airtight container; cover. Store for up to 3 days or freeze for up to 3 months.