In a bowl, whisk together flour, baking soda, salt, and ground spices.
In another bowl with an electric mixer, beat together butter and sugar until light and fluffy.
Add molasses, egg, flour mixture, crystallized ginger, and zest to butter-sugar mixture; beat until a dough forms.
On a sheet of wax paper, pat dough into an 8x5-inch rectangle, about 1-inch thick.
Chill dough, wrapped in wax paper, 4 hours, or until firm. It can be kept in the refrigerator for one week or the freezer for one month. Thaw the frozen dough in the refrigerator until sliceable, about 4 hours.
Preheat oven to 350 degrees F. Lightly butter 2 baking sheets.
Cut dough crosswise into slices about 1/6-inch thick and arrange about 2 inches apart on baking sheets.
Bake bars in batches in middle of oven until deep golden brown, 8-10 minutes. Transfer to racks to cool. The baked cookies keep in airtight containers at room temperature for 4 days.