Preheat the oven to 375°F (190°C).

Line 3 baking sheets with parchment paper.

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

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

Add the large egg; beat until thoroughly 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 until just combined.

Using a spoon, drop 1 tablespoon of dough onto the prepared baking sheets, leaving 2 inches between each cookie. For bigger cookies, scoop out 0.25 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 wire rack to cool completely.

While the cookies cool, prepare the filling. Beat butter with an electric mixer on high speed until light and fluffy, about 1 minute.

Add powdered sugar, 0.5 cup at a time, and continue to mix on low speed until combined.

Pour in the milk and pure vanilla extract, and beat on high speed until you reach desired consistency. Add more milk if necessary for a thinner frosting.

Once the cookies are completely cool, spread frosting onto the flat side of one whoopie pie cookie and place another cookie on top, gently sandwiching them together.