Make the cake: Preheat oven to 350 degrees. Butter two 8-by-2-inch round cake pans, line bottom with parchment, and butter parchment. Dust with cocoa powder, and tap out excess.
Sift flour, cocoa powder, baking powder, baking soda, and salt into a large bowl. Whisk pumpkin, oil, and cream in a medium bowl. Beat butter and sugar with a mixer on medium-high speed until pale and fluffy, 4 to 5 minutes. Add eggs, 1 at a time, beating well after each addition, then add vanilla. Reduce speed to low, and add flour mixture in 3 additions, alternating with pumpkin mixture, starting and ending with flour. Divide batter between pans, and spread evenly.
Bake until testers inserted into centers of cakes come out clean, 30 to 35 minutes. Let cool in pans on wire racks for 15 minutes. Invert cakes onto wire racks, remove parchment, and let cool.
Make the glaze: Place chocolate in a food processor. Bring cream and corn syrup to a simmer in a small saucepan, whisking to combine. Pour over chocolate, and process until smooth.
To assemble the cake: Trim top of 1 cake to create a flat surface. Reserve trimmings for another use. Place cake, cut side up, on a serving plate. Spread chilled pastry cream over cake, leaving a 1/2-inch border. Top with remaining cake, bottom side up, and press gently to spread pastry cream to edge. Stir chocolate glaze, then ladle over top of cake so some drips down sides. Refrigerate until chilled, at least 1 hour or, preferably, overnight. Decorate with candy spiders if desired.