Preheat the oven to 350 and prepare an 8x8-inch cake pan with parchment and grease. Set to the side.
Combine the pumpkin puree, brown sugar, granulated sugar, oil, eggs, vanilla, and buttermilk in a large bowl. Use a whisk to combine.
Add in the flour, baking powder, salt, and spice to the wet ingredients. Stir with a whisk to combine.
Pour the batter into the prepared pan and bake the cake for 25-35 minutes. A toothpick will run clean from the cake when done.
While the cake is baking, make the cinnamon whipped cream. Whip the heavy cream with a hand mixer or in the bowl of a stand mixer with a whisk attachment on medium-low speed until soft peaks begin to form. Gently fold in the powdered sugar, vanilla, and cinnamon to the mixture. Add more or less cinnamon until you are set with the flavor.
When the cake is completely cool, top the cake with the whipped cream. Serve and enjoy.