Preheat the oven to 350 degrees F.
Mix in a bowl the flour, baking soda, salt, ginger, cinnamon and nutmeg. In a stand mixer with a paddle attachment, cream together the butter and brown sugar until light and fluffy. Mix in the molasses, vanilla and egg. With the mixer running, slowly add in the dry ingredients until well combined. Flatten the dough into a disc and refrigerate for 1 hour.
Roll the dough out to a 1/4-inch thickness and cut out in ninja shapes, or as desired. Transfer to a greased baking sheet and bake until the cookies turn light brown at the edges. Cool on a wire rack.
Meanwhile, mix together the confectioner's sugar and 5 tablespoons water in a bowl until an icing consistency is reached. If too thick, add water; if too thin, add confectioner's sugar. Tint with food coloring, if desired. Pipe the icing onto the cookies and decorate as desired.