In a heavy saucepan, combine the brown sugar, butter and molasses. Heat until brown sugar is dissolved.
Dissolve the baking soda in the boiling water and stir into the molasses mixture. The mixture will bubble up. Remove from heat.
Sift the flour and spices together and gradually add to the saucepan, stirring with a wooden spoon until well incorporated. Form the dough into a rectangle, wrap and let set overnight in refrigerator.
The next day, remove from fridge and allow to soften, until easy to roll, about 15 minutes.
Pre-heat oven to 275 degrees F. and line baking sheets with parchment or silpat baking sheets.
On a floured board, roll small pieces of dough as thin as possible. (dough should be thin enough to see one's hand through it.) Cut with cookie cutter and set 1" apart on baking sheets.
Bake for 10-12 minutes. Allow to cool on baking sheets for 2-3 minutes before transferring to wire racks to finish cooling.