Make the crepes: Combine the flour, cocoa powder, sugar, pie spice, salt, milk, melted butter, eggs and vanilla in a blender. Blend, scraping down the sides occasionally, until smooth. Transfer the batter to a bowl; cover and let sit 1 hour.
Make the filling: Whisk the sugar, eggs, cornstarch, pie spice, vanilla and salt in a medium bowl. Combine the milk and pumpkin in a medium saucepan and cook over medium heat, whisking, until steaming. Gradually whisk in the egg mixture and bring to a boil, whisking constantly, until very thick. Strain the filling through a fine-mesh sieve into a medium bowl, pushing it through with a rubber spatula. Place a piece of plastic wrap directly onto the surface and refrigerate 2 hours.
Beat the heavy cream in a large bowl with a mixer on medium-high speed until stiff peaks form. Working in three batches, fold the whipped cream into the filling until combined. Refrigerate until firm, at least 1 hour.
Make the crepes: Line a baking sheet with parchment paper. Heat an 8-inch nonstick skillet over medium heat until hot; lightly brush with melted butter. Add about 3 tablespoons batter and swirl to coat the bottom of the pan. Cook until the crepe is dry on the bottom and set on top, 2 to 3 minutes. Carefully flip and cook 30 more seconds. Invert onto the prepared baking sheet. Repeat with the remaining batter to make about 20 crepes, brushing the pan with more butter and stacking the crepes as you go. (Don't worry if a few tear; they can be layered in the center of the cake.)
Assemble the cake: Lay a crepe on a plate or cake stand and spread with 2 to 3 tablespoons of the pumpkin filling, spreading it to the edges. Top with another crepe and repeat with another layer of the pumpkin filling. Layer the remaining crepes and filling, ending with a crepe. Refrigerate the cake at least 6 hours or overnight.
Make the ganache: Put the chocolate in a small bowl. Heat the heavy cream in a small saucepan over medium heat until steaming, then pour over the chocolate. Let sit 5 minutes, then whisk until melted and smooth. (If the chocolate is not fully melted, microwave in 15-second intervals.) Let sit until thickened but still pourable, about 5 minutes. Pour the ganache over the crepe cake, letting the excess drip down the side, then gently smooth the top with a small offset spatula. Let the chocolate set, about 15 minutes. Run a sharp knife under hot water to slice.
For clean slices, run a sharp knife under hot water and wipe dry before cutting.
Photograph by Andrew Purcell