In a medium saucepan, melt butter over medium-low heat.
Add brown sugar, molasses, salt, cinnamon, ground ginger, and allspice. Stir mixture together until well incorporated. Remove from heat.
Transfer mixture to a medium to large mixing bowl and add the egg and beat it in with the mixture.
In a separate bowl, add baking powder, soda, and flour and whisk together.
Add flour to the molasses mixture and stir together with a wooden spoon.
Divide dough into two equal halves. Over a sheet of plastic wrap, shape dough into a thick large rectangle and wrap each tightly. Refrigerate for at least one hour.
Preheat oven to 350℉. Line cookie sheets with parchment paper and set aside.
On a floured surface, roll dough into 1/4 inch thick rectangle with a floured rolling pin.*
Cut out shapes with gingerbread man cookie cutter or cookie cutter of choice. Place at least an inch apart on cookie sheets.
Bake for 8-9 minutes or until edges are browned. Let cool on cookie sheets for a few minutes and then transfer to cooling racks.
Once completely cooled, frost cookies or sprinkle with powdered sugar.
Store in airtight containers at room temperature or in freezer.