In a saucepan over low heat, combine butter, eggnog, vanilla extract, and nutmeg. Stirring constantly, cook until the butter is melted and it is hot enough to melt the chocolate; approximately 4 minutes. Add 2 cups chocolate chips and continue stirring until the chips are completely melted. Remove from heat and stir in rum. Allow to cool to room temperature. Cover and refrigerate for several hours up to overnight. Overnight is better.
Cover the cookie sheet with parchment paper. Dust hands with powdered sugar and work quickly. Use a small spoon to shape the ganache into balls and place them on parchment paper. Place truffles in the freezer for 45 minutes.
Divide the remaining chocolate in half and place in a small microwavable bowl. Microwave for 1 minute at 50% power. Remove from microwave and stir. Repeat for another minute at 50% power and stir again. If needed, cook for 30 more seconds at 50% power. Be sure and stir completely as the warmth of the bowl melts the chocolate. Stir one teaspoon of cooking oil in while you are stirring to melt the chocolate.
Remove truffles one at a time from the freezer and roll gently in melted chocolate using a fork. Lift the truffle gently out of the chocolate, letting the excess chocolate drip off, and place them on a baking sheet covered with parchment paper or wax paper, letting them gently roll off the fork. Sprinkle with white shimmer sugar. Repeat with half of the ganache balls from the freezer.
Repeat the melting process with the other half of the chocolate and coat the other half of the ganache balls from the freezer. Sprinkle truffles with white shimmer sugar before the chocolate starts to set. Cool completely in the refrigerator.
Store in refrigerator in airtight containers.