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Preheat oven to 350°F. Grease a 9x13-inch pan and set aside.
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For the crust, mix together with a wooden spoon the yellow cake mix (keep 1 cup reserved for later), egg, and melted butter. Once a dough is formed, evenly spread the dough with a metal spoon and press firmly into the bottom of the 9x13 pan.
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For the filling, add the pumpkin puree, pumpkin pie spice, brown sugar, eggs, and milk into the bowl of a stand or hand mixer. Mix on low speed for 30 seconds, then mix on medium speed until well combined (1-2 minutes). Pour pumpkin filling on top of the crust layer and spread evenly.
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For the crumb topping, first slice the cold butter into cubes. Add the 1 cup of reserved yellow cake mix and the sliced butter into a medium bowl. Using a pastry cutter or 2 knives, knead the butter with the cake mix until a crumb-like mixture forms (the butter should be in tiny pieces). Evenly sprinkle the crumb mixture on top of the pumpkin layer.
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Bake for 50-55 minutes or until the crust becomes golden brown. Don’t use a toothpick to test the doneness, since the pumpkin layer will still be wet.
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Once baked, let the cake cool on the stove for 10 minutes, then transfer to a wire rack and continue cooling until it reaches room temperature.
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Cover and let chill in the refrigerator for at least 1 hour before serving. Top with whipped cream and chopped pecans/walnuts, if desired.