Preheat the oven to 350°F (180°C), grease an 8-inch (20 cm) round pan, and line it with parchment paper.
In a medium bowl whisk together the flour, cocoa powder, baking soda, salt, and spices.
In a large bowl beat the egg, granulated sugar, brown sugar, and vanilla until creamy, then mix in the oil and pumpkin puree.
Add the dry mixture into the wet mixture on low speed just until combined, then fold in the chopped chocolate.
Pour the batter into the prepared pan and bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.
Cool the cake completely, then make the glaze by melting the chocolate and cream together in a heatproof bowl over simmering water.
Pour the glaze over the cake, letting it drip down the sides, then smooth with a spatula.
Melt the white chocolate, pipe a spiral on top, and drag a toothpick outward to form the spiderweb, then add cookie crumbs and the mini cookie spider.