Preheat oven and pan: Preheat the oven to 350°F and grease a 9x13 inch baking dish so the crust releases cleanly after baking.
Mix the crust: In a bowl, combine the cake mix, melted butter, and 1 egg until a thick, sticky dough forms. It should hold together when pressed.
Press into pan: Press the dough evenly into the bottom of the prepared pan using damp fingers to keep it from sticking.
Beat the cream cheese: In a separate bowl, beat the softened cream cheese until smooth and free of lumps, about 1-2 minutes.
Add pumpkin and eggs: Mix in the pumpkin puree, remaining 2 eggs, vanilla extract, and pumpkin pie spice until fully combined and smooth.
Sweeten the filling: Add the powdered sugar a cup at a time, beating after each addition, until the filling is smooth and pourable.
Pour over crust: Pour the pumpkin filling over the crust and spread it to the edges with a spatula so it bakes evenly.
Bake until set: Bake for 40-45 minutes, until the edges turn golden and the center still jiggles slightly when the pan is nudged.
Cool completely: Let the cake cool to room temperature in the pan, about 1 hour, before moving to the next step.
Chill before slicing: Refrigerate the cooled cake for at least 2 hours so the filling firms up fully before cutting into squares.
Dust and serve: Dust the top with powdered sugar just before serving and cut into 16 squares with a sharp knife.
