Preheat your 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 well incorporated.

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

Mix in the molasses, then the remaining dry ingredients, and finally the buttermilk, mixing until just combined.

Using a spoon, drop 1 tablespoon of dough onto the prepared baking sheets, leaving about 2 inches between each cookie. For larger cookies, use 0.25 cup of dough.

Bake the cookies for 12 to 15 minutes, or until a toothpick inserted into the center of a cookie comes out clean. Baking time may vary depending on cookie size.

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

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

Gradually add the powdered sugar, about 0.5 cup at a time, continuing to mix on low speed until fully combined and smooth.

Pour in the milk and pure vanilla extract, then beat on high speed until you reach your desired consistency. Add more milk if the frosting is too thick.

Once the gingerbread cookies are completely cool, spread a generous amount of frosting onto the flat side of one cookie and top with another cookie, sandwiching them together.