Use stencils to decorate gingerbread circles and squares with powdered-sugar silhouettes of witches, pumpkins, and spiders.
Heat oven to 350 degrees. Lightly spray one 9-by-13-inch pan and one 8-by-8-inch pan with cooking spray. Line the pans with parchment paper. Combine the water with the baking soda, and set aside. In a large mixing bowl, sift together the flour, spices, and baking powder and set aside.
In another large bowl, cream the butter, add the sugar, and beat on a medium-low speed until fluffy. Beat in the molasses and baking-soda mixture until well combined. Beat in the flour mixture on low speed until combined. Add eggs, and beat well.
Pour batter to equal heights in pans. Bake 15 minutes, and rotate pans in oven to opposite shelves. Bake 10 minutes or until a toothpick inserted into center comes out clean.
Let stand on wire rack until just cool enough to unmold, 20 to 30 minutes. Unmold onto clean piece of parchment paper. Carefully peel off paper from bottom. This smooth surface will be used for stenciling. Let cool completely. (May be made up to this point a day ahead, tightly wrapped, and stored at room temperature.) Use cookie cutter to cut out shapes. Place stencil on gingerbread. Sprinkle sanding or confectioners' sugar over stencil; remove stencil. Transfer to a serving platter. Repeat until all the gingerbread is used.