Preheat oven to 350°F. Grease a 9x13-inch pan and set aside.
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.
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.
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.
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.
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.
Cover and let chill in the refrigerator for at least 1 hour before serving. Top with whipped cream and chopped pecans/walnuts, if desired.
This cake can be saved for 2-3 days. If the cake is left in the refrigerator overnight while covered with plastic wrap or tinfoil, it may cause condensation and the crust will become soggy. I’d recommend covering the cake and leaving it on the counter at room temperature, then chill 1-2 hours before serving.