In a large saucepan over medium-high heat, bring the brown sugar, molasses, and butter to a boil. Remove from the heat and let cool slightly. Dissolve the baking soda in ⅓ cup of water and add it to the molasses mixture. Add the remaining ingredients and stir until well blended. Wrap the dough in plastic wrap and chill for at least 8 hours. Preheat the oven to 350 degrees F. Lightly grease a cookie sheet. Roll out the dough to a ⅛-inch thickness on a floured board. Cut with a gingerbread-man cookie cutter. Bake for 12 to 15 minutes, or until crisp and golden. Allow the cookies to cool before decorating.
Combine all of the ingredients in a small bowl and beat with a fork until smooth. Decorate as desired. Allow frosting to dry before stacking cookies. Makes enough to frost bout 3-½ dozen small cookies.