Preheat your oven to 350°F (180°C) or 160°C if you have a fan oven, and grease a 9x13 inch (approx 23x28 cm) pan. Set aside.
In a large bowl, using a hand mixer, mix together the yellow cake mix, egg, and melted butter.
Press this mixture into the bottom of the prepared baking pan.
For the pumpkin layer, in a separate bowl, using a hand mixer mix together the pumpkin puree and cream cheese until well combined. Then add the eggs, melted butter, cinnamon, nutmeg, salt, and vanilla and mix until smooth.
Gradually add the poswered sugar and mix until well combined. Pour this mixture over the cake layer in the pan.
Bake for 40-50 minutes. The center should be a bit gooey and jiggly, so don't overbake.
Let it cool completely before cutting into squares. Top with whipped cream and serve.