Preheat the oven to 375.

Line 3 baking sheets with parchment paper.

In a medium-sized mixing bowl, whisk together the flour, salt, baking soda, and spices; set aside.

In a large mixing bowl combine the shortening, butter and brown sugar; beat until the mixture is smooth and fluffy.

Add the egg; beat until incorporated.

Add half of the flour mixture to the butter mixture and mix to combine.

Mix in the molasses, the remaining dry ingredients, and the buttermilk.

Using a spoon, drop 1 tablespoon of dough, leaving 2 inches between, on the prepared baking sheets. For bigger cookies, scoop out 1/4-cup of dough.

Bake the cookies for 12 to 15 minutes, or until a toothpick inserted in the center of the cookie comes out clean.

Remove from the oven and place the pan on a rack to cool completely.

Beat butter with mixer on high until light and fluffy, about 1 minute.

Add powdered sugar, 1/2 cup at a time, and continue to mix on low until combined.

Pour in the milk and vanilla, and beat on high until you reach desired consistency. Add more milk if necessary.

Spread frosting onto the cooled whoopie pie and place another cookie on top, and sandwich together.