Step 1

In a large bowl, cream butter and brown sugar until light and fluffy. Beat in egg yolks, eggnog and rum extract. Gradually mix in the flour. Chill for two hours.

Step 2

Preheat oven to 325°. Line baking sheets with parchment paper. Shape into 1.5 inch balls and bake for 10-12 minutes or until bottoms are golden brown (tops won't brown much). Cool completely.

Step 3

For the Frosting:  Beat In a large bowl, beat the first five frosting ingredients until blended; Mix in additional eggnog to reach desired consistency. Frost cookies and sprinkle with nutmeg if desired. Store in airtight containers.