Preheat the oven to 350° F. Line 12 holes of a muffin tin with paper cupcake liners.
Sift together the flour, baking powder, baking soda, cinnamon and salt. Mix together with a fork, then set aside.
Beat the butter and sugar together until lightened and creamy. Add the eggs, one at a time, beating well after each. Make sure to scrape down the sides of the bowl every now and then.
Add the vanilla and pumpkin and mix through to combine.
Add 1/3 of the flour mix and stir through until just combined. Follow with half the buttermilk and mix through. Repeat this process until all the flour and buttermilk. Make sure not to overmix at this point or the cupcakes may turn out dense.
Split the batter into 2 bowls. Add the cocoa to one of them and gently stir through to combine.
Now add the plain pumpkin batter back to the chocolate batter and just turn it over once or twice with the spoon to start to marble it.
Divide the batter up amongst the 12 cupcake liners.
Bake for around 20 minutes, turning the trays half way through so they bake evenly. A toothpick should come out with just a crumb or two clinging to it when they are done.
Pour half batter into a separate bowl, add cocoa and mix through