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 and whisk until smooth.

In a mixing bowl, beat butter, brown sugar, granulated sugar, and vanilla until fluffy. Add eggs one at a time and mix well.

Alternately add the flour mixture and pumpkin mixture to the wet ingredients, beginning and ending with the flour mixture. Mix until just combined.

Divide batter evenly between prepared pans. Bake for 18-26 minutes or until a toothpick inserted comes out clean.

Let cakes cool completely before frosting.

Prepare whipped cream by whipping heavy cream until soft peaks form. Gradually add confectioners' sugar and fold in pumpkin puree, cinnamon, and allspice.

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

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