Cream together the butter and brown sugar until light and fluffy. Meanwhile, mix flour, ginger, cinnamon, baking soda, nutmeg and salt in a large bowl and set aside. Add molasses, egg and vanilla to the creamed mixture on a moderate speed until well mixed. Gradually beat in the flour mixture on low speed until well mixed. Press the dough into a thick flat disk and wrap in plastic wrap for at least 4 hours or overnight.