Place 24 paper baking cups in 2 (12-cup) muffin pans; spoon batter into cups.
Bake at 350 for 18 to 20 minutes or until a wooden pick inserted in center comes out clean.
Transfer to wire racks; cool completely. Beat butter and cream cheese with an electric mixer until creamy. Gradually add powdered sugar alternately with eggnog, beginning and ending with sugar and beating at low speed.
Add freshly grated nutmeg and vanilla extract; beat until smooth.
