Preheat the oven to 350°F. Spray the tombstone cake pan with baking spray for best results (see notes.)
In a small bowl, whisk together the buttermilk, egg and vanilla. Set aside.
In a separate bowl, combine the flour, baking soda and activated charcoal.
In a mixer or in a large bowl with a hand mixer, beat the butter on medium high speed until pale and fluffy. Add the granulated sugar and cream together on medium speed for 1 minute. Add the pumpkin puree to the bowl and beat into the butter and sugar. It'll look a little strange, but everything will come together after adding the remaining ingredients.
With the mixer on low speed, add half the amount of dry flour mixture to the bowl, then half the buttermilk mixture. Mix until just combined, about 1 minute. Repeat with the remaining ingredients. Scrape down the sides of the bowl and mix on medium speed until combined, about 30 seconds to 1 minute.
Divide the batter equally in the prepared pan. Bake at 350°F for 20 to 25 minutes or until a toothpick inserted in the center of a cake comes out clean. Let the tombstone cakes cool in the pan for 5 to 10 minutes before inverting onto a cooling rack. Cool the cakes completely before adding decorative frosting.
For the icing: In a small bowl combine the confectioners sugar and vanilla. Slowly add the heavy cream until a paste like consistency forms. You want it thick so it will set and harden but soft enough to push through a piping bag using a Wilton Tip#3.