In a mixer, cream together the butter and the sugar until light and fluffy. Scrape down the sides of the bowl with a rubber spatula, then beat the egg. Cream until the mixture is light and lemon-colored. Add the molasses and mix until combined. Scrape down the side of the bowl.
In a separate bowl, stir together the salt, ginger, cinnamon, and baking soda. Add this to the butter mixture and mix until combined. A third at a time, add the flour to the butter mixture and mix just until combined. Wrap in plastic and chill at least 2 hours or overnight.
Preheat the oven to 350 degrees F and lightly grease two baking sheets. Roll the dough into balls 1/2 to 2 inches in diameter (depending on what size you like them) and arrange them in well-spaced rows on the cookie sheets. Bake about 10-12 minutes, just until crisp. The cookies will be thin and flat. Let cool on the baking sheet 2 minutes, then transfer to wire racks. When cool, store in an airtight container.