Spray the interior of a 6-inch springform pan with cooking oil spray. Line the bottom with a 7-inch parchment round (I like using a bigger round at the bottom because it will make it easier to transfer the cake to a plate later). Also line the inner sides of the cake pan with strips of cut parchment paper.

Add chocolate chips and butter to a microwave safe bowl. Microwave in 30-second intervals, stirring in between with a spatula, until the butter and chocolate are completely melted and smooth. Alternatively, you can do this over low heat in a double boiler on the stove.

Transfer the chocolate to a blender. Add in the pumpkin puree. Blend until completely smooth.

Pour the mixture into the prepared cake pan and smooth the top with an offset spatula or a butter knife.

Put the cake in the fridge to set about 2-4 hours. Once cake is set, carefully release it from the cake pan and peel off the parchment paper on the sides. You can use the excess edges of the bottom of the parchment paper round to help move the cake onto a plate. I also sometimes just leave the parchment and the bottom cake pan piece in place to make it easier to serve.

To achieve smooth cut slices, heat the blade of a large sharp knife with warm water. Wipe the blade dry and then cut into your cake, wiping down the knife after each cut and reheating the knife as needed. After the cake slices are cut, I also like to run the knife blade back up the side of the slices, which really smooths the cut edges.