Special equipment: a 9-inch round cake pan
1. Position 2 racks in the center of the oven, and preheat to 200°C.
2. Stir together the granulated sugar and 1 teaspoon of the cinnamon in a small bowl.
3. Let the dough rounds soften at room temperature for 10 to 15 minutes, then unroll onto 2 baking sheets. Brush both sides of the dough rounds with the melted butter, and sprinkle the tops with the cinnamon sugar.
4. Bake the dough rounds, rotating the baking sheets halfway through, until they are light golden brown and crunchy, about 15 minutes. Let cool completely on the baking sheets.
5. While the dough bakes, spread the walnuts out on another baking sheet, and bake until toasted, about 8 minutes. Let cool, then finely chop.
6. Meanwhile, beat 3 cups of the heavy cream, the pumpkin pie puree and 3 tablespoons of the confectioners' sugar with an electric mixer in a large bowl until stiff peaks form, about 5 minutes.
7. Put 1 of the baked dough rounds on a cutting board, and center a 9-inch round cake pan on top of it. Use the cake pan as your guide to trim the dough with a knife into a 9-inch circle. Reserve the scraps in a bowl. Repeat with the remaining dough round. Crumble the scraps into bite-sized pieces.
8. Line the cake pan with plastic wrap, leaving a 2-inch overhang on all sides. Spread a third of the pumpkin mixture on the bottom of the pan, followed by 1 of the baked dough rounds.
9. Top with half the remaining pumpkin mixture, then the crumbled dough scraps. Finish with the remaining pumpkin mixture and dough round. Cover tightly with plastic wrap, and refrigerate for 12 hours or overnight.
10. When ready to serve, unwrap the plastic wrap, and invert the cake onto a platter. Remove the cake pan and plastic wrap. Press the chopped walnuts into the sides of the cake. Beat the remaining 1/2 cup heavy cream, 1 tablespoon confectioners' sugar and 1/4 teaspoon cinnamon together until soft peaks form; slather the mixture on top of the cake. Cut into slices with a serrated knife.