Combine cocoa, flour, sugar, baking soda, baking powder and salt into the bowl of a mixer. Beat on low until combined.
Add egg, buttermilk, water, oil and vanilla. Increase speed to medium and beat until very smooth, about 3 minutes.
Pour into cocoa prepared pan. Bake until set about 30-35 minutes. Let cool completely before turning out.
When both layers are fully cooled, take the one pumpkin layer and cut it in half, then the one chocolate layer and cut it in half so you have 4 sections of cake. Frost with Whipped Brown Sugar Icing alternating one chocolate section, pumpkin, chocolate and finish with pumpkin.