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 1-1.5 inch balls (they spread a lot while baking,) roll the dough balls in the sugar mixture and place on a cookie sheet.
Pop all the dough into the freezer for about 10-15 minutes before baking, this will yield a thicker cookie. Bake for 9-10 minutes or until the tops are crackled and the edges are set.