Preheat the oven to 350 F. Lightly grease a 9x13-inch baking dish.
In a large mixing bowl, combine the yellow cake mix, 1 egg, and ½ cup melted butter, and mix well. Transfer the batter to the greased baking dish and press into an even layer.
In another large bowl, beat together the cream cheese and pumpkin until smooth. Add 3 eggs, the vanilla, and the other ½ cup of melted butter, and beat well.
Slowly add the powdered sugar, followed by the pumpkin pie spice and salt. Mix thoroughly.
Spread the cream cheese mixture over the cake batter layer, and bake for 40 to 50 minutes. The center should be slightly gooey when removed from the oven.
Let the cake cool completely before serving with fresh whipped cream, if desired.