I vote for butter. See inside
If you want a spectacular ginger cookie that gets really hard, try this one from Michael. I've baked them 4 times so far and have never eaten a better ginger cookie. Michael forms the dough for cookies, I roll the dough, refrigerate then cut and bake putting a little sugar on top before popping in the oven. I believe the success of the hardness is due to not only the butter but also that there is some baking soda in the cookie.
Michael's Spicy Ginger Cookies
1 c butter
1 c sugar
1 c molasses
2 egg yolks
4 c all-purpose flour
1 tsp salt
1 tsp baking powder
1 tsp baking soda
1 tsp ground cinnamon
1-½ tsp ground cloves
1 T ground ginger
1 tsp ground nutmeg
Cream butter and sugar. Stir in molasses and egg yolks. Mix in the dry ingredients/ Cover and chill. Roll into two logs and refrigerate until hard. Slice and bake at 350 10 minutes.