Heat oven to 350°F. Butter and flour a 9-by-13-inch cake pan; set aside. In a bowl, combine boiling water and baking soda; set aside.
In a large bowl, sift together flour, ground spices, salt, and baking powder; set aside.
In an electric mixer with the paddle attachment, cream butter until light. Beat in brown sugar until fluffy.
Beat in molasses, chopped and grated ginger, baking-soda mixture, and flour mixture. Beat in eggs.
Pour batter into prepared pan; bake until a toothpick inserted in center comes out clean, 30 to 35 minutes.
Let cool on a wire rack. Cut into squares; dust with confectioners' sugar.
