In a large bowl, cream together 1 cup margarine and sugar until smooth. Beat in the egg and molasses until well blended. Dissolve the baking soda in boiling water; stir into the molasses mixture. Gradually mix in the flour to make a smooth dough. Cover and chill dough for at least 1 hour.
Preheat oven to 350 degrees F (175 degrees C). On a lightly floured surface, roll the dough out to 1/8 inch in thickness.
Cut into circles using a 3 inch round cutter.
Cut a smaller 1 inch circle out of half of the cookies. It is easier to cut the center hole after the cookies have been placed onto the cookie sheets.
Place cookies 1 inch apart onto ungreased cookie sheets.
Bake for 7 to 10 minutes in the preheated oven, until edges begin to darken. Allow cookies to cool completely before sandwiching. Frost solid cookies and place a cookie with the center hole on top.
To make the frosting: Beat 1 tablespoon of margarine and water together. Blend in the ginger and salt. Gradually beat in the confectioners' sugar until the desired consistency is reached. Sandwich between cooled cookies.
