1. Preheat the oven to 375°F. Lightly grease three baking sheets, or line them with parchment paper.
2. In a medium bowl, whisk together the flour, salt, baking soda, and spices; set aside.
3. In a large mixer bowl fitted with a paddle, combine the shortening, butter and brown sugar; beat until the mixture is smooth and fluffy.
4. Add the egg, beating until incorporated; stop the mixer and scrape the sides and bottom of the bowl.
5. Add half the dry ingredients, mix to combine, then mix in the molasses, the remainging dry ingredients, and the buttermilk.
6. Once the dough is made, scoop by the 1/4-cup, 6 to a baking sheet, and bake for 15 minutes, until a pick inserted in the center of each cookie comes out clean. Remove from the oven and place the pan on a rack to cool completely before removing from the baking sheet.
7. For the filling: Place the cream cheese and confectioners' sugar in a mixing bowl. Mix at low speed until the mixture begins to come together.
8. Scrape the bowl, add the vanilla, and continue mixing until the filling is smooth.
9. Add the crystallized ginger, if using, and adjust the consistency of the filling with a little milk, if needed.
10. To assemble: Spread a generous 2 tablespoons of filling on the bottom of one cookie.
11. Top with another cookie and press together gently to make the whoopie pie.