Mix flour, sugar and Xanthan gum. Add butter and mix into crumbs with hands. Reserve 1/2-cup crumbs and set aside. Mix hot water and baking soda and add to crumb mixture with molasses.
Pour into greased 8 x 8-inch pan. Sprinkle reserved crumbs over top. Bake in preheated 350°F oven for about 30 to 35 minutes or until toothpick tests clean. Cool before cutting.