Preheat oven to 350°F. Mix flour, baking powder, ginger, cinnamon and baking soda in medium bowl. Set aside. Beat butter, brown sugar and molasses in large bowl with electric mixer on medium speed until well blended and smooth. Add egg and vanilla; beat well. Add flour mixture alternately with boiling water, beating on low speed after each addition.
Spoon batter into greased 9-inch square baking pan.
Bake 40 minutes or until toothpick inserted into center comes out clean. Cool in pan on wire rack 10 minutes. Invert cake onto wire rack; cool completely.