Cream together the butter and sugars until smooth and fluffy. Add the egg and rum (or extract,) and mix together until smooth.
Add the dry ingredients and mix on low until well combined.
Roll dough into 1 inch balls. Roll the dough balls in the sugar mixture. Chill for 30 minutes, then place dough balls on a parchment lined cookie sheet, 2 inches apart.
Bake for 9-10 minutes or until the tops are crackled and the edges are set.