Preheat oven to 350°F. Prepare two 8x8-inch baking pans with butter and flour or non-stick spray.

In a medium bowl, sift together flour, cocoa powder, baking powder, baking soda, and salt.

In a small bowl, combine buttermilk and pumpkin puree. Whisk until smooth.

Beat the butter, sugars, and vanilla in a mixer until light and fluffy. Add eggs one at a time, mixing well after each addition.

Gradually add the flour mixture and buttermilk mixture to the wet ingredients, beginning and ending with the flour mixture. Mix on low until combined.

Divide batter evenly between prepared pans and bake for 18-26 minutes, until a toothpick inserted comes out clean.

While cakes are cooling, whip the heavy cream to soft peaks. Fold in pumpkin puree, cinnamon, and allspice. Gradually add confectioners' sugar to sweeten.

Prepare ganache by heating heavy cream until boiling, then pour over chocolate chips. Stir until smooth and shiny.

Assemble the cake by layering with whipped pumpkin cream and drizzling with ganache. Garnish with chocolate jimmies if desired.