Whisk together the flour, baking powder, salt, sugar, and spices. Add the pumpkin, milk, butter, and vanilla. Stir just till blended. Spread into a well-greased round casserole dish or 9″ square pan. (I tested in my Staub 10.5 by 7.5-inch casserole dish and it was perfect).
Mix together the brown sugar, cinnamon, and walnuts in a small bowl. Sprinkle evenly over the batter. Pour hot water over the top. DO NOT STIR and trust the process.
Bake at 350° for about 50-55 minutes (If it begins to brown, cover the top with foil for the last 10 minutes of cooking). The cake will rise to the top, it should be cooked through, but there will be a little sauce in the middle and at the bottom. It’s meant to be ooey and gooey
Serve warm with vanilla ice cream or dairy-free ice cream.