In small bowl, mix lime zest together with sugar. Set aside.
Separately, using hand mixer, beat butter in mixing bowl 'til creamy.
Add 1 cup of lime sugar and beat together with the butter.
(Remaining 1/2 cup lime sugar will be used to roll dough balls in before baking.)
Add in egg and molasses and stir 'til smooth.
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 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 may still be slightly puffy.
Remove from 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.