Beat butter in large bowl with electric mixer at medium speed until smooth.
Add brown sugar; beat until well blended.
Heat milk in small saucepan over low heat; add coffee granules, stirring to dissolve.
Add milk mixture, eggs, rum extract and vanilla to butter mixture. Beat at medium speed until well blended.
Combine flour, baking powder, nutmeg and salt in large bowl. Gradually add flour mixture to butter mixture, beating at low speed after each addition until blended.
Shape dough into 2 logs, about 2 inches in diameter and 8 inches long (dough will be soft; sprinkle lightly with flour if too sticky to handle).
Roll logs in chocolate sprinkles, coating evenly, or leave rolls plain and dip in chocolate after baking. Wrap each log in plastic wrap; refrigerate overnight.
Preheat oven to 350°F Grease cookie sheets.
Cut into 1/4-inch-thick slices; place 1 inch apart on cookie sheets.
Bake 10-12 minutes or until edges are lightly browned. Transfer to wire rack to cool. Dip plain cookies in melted semisweet or white chocolate.