Preheat oven to 350°F (175°C). Grease and line three 8-inch round cake pans.
In a large bowl, whisk together flour, baking soda, cinnamon, ginger, nutmeg, and salt.
In a separate bowl, mix both sugars, oil, eggs, and vanilla until smooth.
Add pumpkin puree and mix until combined.
Gradually alternate adding the flour mixture and buttermilk, beginning and ending with flour.
Pour one-third of the batter into a separate bowl and stir in the cocoa powder.
Divide the plain pumpkin batter evenly into the cake pans. Spoon the chocolate batter over each and gently swirl with a knife.
Bake for 30–35 minutes, or until a toothpick inserted comes out clean. Let cool completely before frosting.