Gather the ingredients. Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13 (moms says 8x8 but will have leftover filling) -inch baking dish.
Mix pumpkin purée, evaporated milk, sugars, eggs, and spices together in a bowl until well combined; pour into the prepared dish.
Sprinkle cake mix over top, then drizzle with melted butter and top with walnuts.
Bake in the preheated oven until a knife inserted near the center comes out clean, about 1 hour. Let cool completely before cutting into 24 squares.
