Whisk together the flour, baking powder, salt, sugar, and spices. Add the pumpkin, oil, and vanilla. Stir just till blended. Spread into a well greased round casserole dish or 9″ square pan.
Mix together the brown sugar, cinnamon, and walnuts in a small bowl. Sprinkle evenly over the batter. Pour hot water over the top.
Bake at 350° for about 50-60 minutes. The cake will rise to the top, it should be cooked through, but there will be sauce at the bottom. Serve warm with vanilla ice cream.