Combine pumpkin with the brownie mix in a large bowl and stir until smooth (batter will be very thick, but don't add anything else.)
Spray a square baking pan (9" X 9" works best) with nonstick spray and pour in the mixture. Spoon peanut butter on top and use a knife to swirl it around the top of the batter. Bake in the oven for 35 minutes. (The batter will remain very thick and fudgy, and it should look undercooked.) Allow to cool.
Cover pan with foil and place in the fridge for at least 2 hours. Cut into 36 squares.