Heat oven to 350 deg. Spray oven-safe 11-inch skillet or a 9 X 13 cake pan with nonstick cooking spray.
Prepare cake mix according to package directions, except reduce the water by 1/4 cup. After mixing, fold in the mincemeat and nuts.
Bake 35 to 40 min. Cake is done with toothpick inserted in center of cake comes out clean.
Let cool 5 min. Run a knife around edge of pan and invert onto serving plate.
Sauce: Combine butter, sugar and eggnog in small saucepan. Bring to a boil. Boil for 2 min. Remove from heat stir in rum or rum extract and a dash of nutmeg. Serve warm.