Preheat the oven to 350°F.
Coat a 9 x 13 baking dish with butter, then sprinkle 1 1/2 tablespoons of spice cake mix evenly over the butter.
1 tbsp. butter, softened, 1 (15 oz.) box spice cake mix
In a bowl, mix together the pumpkin purée, brown sugar, milk, heavy cream, eggs, cinnamon, and pumpkin pie spice until combined. Set aside.
1 (15 oz.) can pumpkin puree, 3/4 cup light brown sugar, 1/2 cup milk, 1/2 cup heavy cream, 3 large eggs, room temperature, 1 tsp. ground cinnamon, 1 tsp. pumpkin pie spice
Transfer the pumpkin mixture to the baking dish and spread it evenly. Top with the remaining cake mix, followed by the pecans and sliced butter.
1 cup chopped pecans, 12 tbsps. cold butter, sliced
Bake for one hour or until the cake is set. Remove from the oven and place on a cooling rack.
To make the whipped cream, place a metal or glass bowl in the fridge for 20 minutes. Once chilled, add the whipped topping ingredients and mix for 60 to 90 seconds on medium speed. Scrape down the sides and mix for an additional 30 to 90 seconds until stiff peaks form.
2 cups cold heavy cream, 1/2 cup confectioners' sugar, 1 tsp. vanilla extract