Heat oven to 325 degrees. Butter an 8-inch springform pan. Place a small saucepan over medium-low heat, then add butter. Let heat, stirring with a spoon once or twice, until butter melts. Turn off the heat. In a large bowl, whisk together sugar, cocoa and salt. Pour melted butter into the cocoa mixture and whisk until smooth. Let mixture cool until lukewarm, about 5 to 10 minutes. Whisk pumpkin purée into the cooled cocoa mixture, then whisk in the eggs, one at a time. Whisk in flour until smooth and silky looking. Pour the batter into prepared pan, scraping the bottom of the bowl with a rubber spatula. Bake until the top of the cake is firm and doesn’t wiggle if you shake the pan, 30 to 40 minutes. Using oven mitts, transfer pan to a wire rack to cool completely, at least 2 hours. Make the cream: In a large mixing bowl using electric beaters or an electric mixer fitted with the whisk, beat the cream and 1 tablespoon confectioners’ sugar until thick and fluffy, about 2 minutes. Store whipped cream in the fridge for up to 6 hours if not using immediately. Just before serving, in a small bowl, vigorously whisk the remaining 3 tablespoons/25 grams confectioners’ sugar, pumpkin and vanilla until the mixture is smooth and no white lumps of sugar remain, about 1 minute. Using a rubber spatula, gently fold in the pumpkin mixture into the whipped cream until very streaky. Run a small offset metal spatula or butter knife around the inner edges of the cake pan, then release the sides of the cake pan and remove them. Place the torte, still on the bottom of the pan, onto a platter. Dollop the pumpkin-streaked whipped cream on top and sprinkle lightly with pumpkin pie spice or cinnamon.