Preheat oven to 350 degrees F. and prepare a baking sheet by lining it with parchment.
In the bowl of a mixer, cream the shortening with the salt, molasses and sugar for about 3 minutes.
Add the egg and mix until light about 4 minutes.
Sift the flour, ginger and baking soda together into a bowl.
Gradually add the dry ingredients to the mixer, scraping down the bottom and sides of the bowl as needed.
Divide the dough into fourths and on a well-floured surface, roll the dough to between 1/8 - 1/4-inch in thickness. With a doughnut cutter, cut half of the dough and use a round cookie cutter the same size for the other half of the dough.
Place cookies onto prepared baking sheet.
Repeat until all the dough has been cut.
Bake for 10 - 12 minutes.
Place on wire racks to cool.
In a mixing bowl, mix the butter, sugar, cinnamon, nutmeg, and salt together.
Add the boiling water one tablespoon at a time until smooth and creamy, scraping down the bottom and sides of the bowl, if necessary.
Spread the round cookies with a little bit of filling and then place a doughnut type cookie on top. Gently press.