Beat butter with mixer 'til creamy.
In small bowl, mix lime zest together with sugar.
Add 1 cup of lime sugar to a mixing bowl.
(Remaining 1/2 cup lime sugar will be used to roll dough balls in before baking.)
Add in egg and molasses.
Separately, sift together (or mix well with a fork): flour, ginger, baking soda, cinnamon and salt.
Stir flour mixture into wet ingredients.
Form into 1 1/2 inch round dough balls.
Roll dough balls in remaining lime granulated sugar.
Place 2 inches apart on ungreased baking sheet or parchment paper.
Bake 8 minutes @ 350 degrees or 'til tops are round and cracked. They might still be slightly puffy.
Remove from the oven and whack the pan hard on the counter or stovetop. (Yup.) The middle will fall and they'll be flat and crackly. Just like a good gingersnap ought to be.