In a medium-size bowl, combine the sugars and butter. Add in the pumpkin and mix well. Add the salt, cinnamon, pumpkin pie spice and baking soda and mix until combined. Add the flour and mix just until combined. The batter will be thick. Lastly, fold in the cinnamon chips.
Spray a 9" round baking pan with cooking spray. Spread the pumpkin batter evenly on the bottom of the pan. Sprinkle the chopped pecans on top of the batter and press down lightly with your hand so they stick in the batter.
Bake the cookie for 20-23 minutes, until it is cooked through. As soon as the cookie pie comes out of the oven, use a flat bottomed glass cup to press very lightly on top of the cookie. This will give you an even top.
While the cookie pie is cooling, prepare the buttercream. In a small bowl, use a spoon to mix the butter well for 2-3 minutes. It should get creamy and whitish in appearance. Add in the vanilla, cinnamon and powdered sugar and mix. Pour in 1-2 tablespoons of heavy cream, until the buttercream is the texture you would like it.
Use a decorating bag with a large tip (Wilton 1M or 2D) to decorate the edges of the cake or just spread the buttercream overtop the whole cookie. If you'd like to do this, I would recommend doubling the buttercream recipe to give you a little bit more frosting to work with.