1In a large saucepan, whisk together eggs,sugar and cocoa powder until well combined. Whisk in milk.
Add cinnamon sticks, nutmeg, mace and cloves; place over medium heat and whisk gently for about 10 minutes or until mixture has small bubbles around the edge of pan and thermometer reaches 160 F (71 C).
2Remove from heat and pour through fine mesh strainer into a large bowl. Whisk in rum (if desired) and cover with plastic wrap, directly on surface, and refrigerate for at least 4 hours or up to 2 days.
3In a bowl, beat whipping cream until stiff peaks form. Dollop each cup of egg nog with whipped cream and sprinkle with cinnamon or chocolate shavings, if desired.