Preheat oven to 350 degrees and spray two 8” round cake pans with non-stick cooking spray. Set aside. In a large bowl, combine brownie mix, brown sugar, baking powder, baking soda, salt and flour. Set aside.
In a medium sized bowl, whisk eggs then slowly add melted and cooled butter and mix until thickened. Add pumpkin and buttermilk and stir to combine. Add the wet ingredients to the dry ingredients and whisk until just combined and smooth. Do not over mix.
Divide batter evenly into the prepared pans. Bake 25 - 30 minutes or until a toothpick inserted in the center comes out clean. Remove from oven and cool 10 minutes before removing cakes from pans. Cool completely on a wire rack.
To make the topping, use an electric hand blender to whip heavy cream to soft peaks, about 2-3 minutes. Slowly add the dry frosting mix and incorporate with the blender on low to medium speed. Stir together the cinnamon and pumpkin, then fold mixture into the whipped cream.
To assemble, use a serrated knife to level each cake, removing the rounded dome. Place half of the whipped cream mixture onto the surface of one cake, then top with the second cake and remaining whipped topping. Serve immediately or refrigerate until served.