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.
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.