Whisk flour, baking powder, salt, sugar, cinnamon, cloves, ginger, and nutmeg in a large bowl.
Add canned pumpkin, vegetable oil, and vanilla. Stir just until combined.
Spread thick batter evenly in the prepared pan.
Mix brown sugar and cinnamon for the topping and sprinkle over the batter.
Add nuts if using.
Pour hot water evenly over the top. Do not stir.
Bake until the cake is set on top and sauce forms underneath.
Let rest 10 minutes before serving.
Serve warm with whipped cream or ice cream.
Notes
Do not stir after pouring hot water over the topping; the water creates the sauce underneath. Use the correct pan size so the self-saucing layer works properly. Serve warm for the best texture. Leftovers keep refrigerated and reheat well in the microwave.