Preheat oven to 350°F/175°C. Grease and line a 9×13 inch pan with parchment paper.
In a medium bowl, sift together the flour, cocoa powder, baking soda, baking powder, sugar and salt and mix until combined.
In another bowl, add the buttermilk, oil, eggs and vanilla and whisk to combine.
Pour the wet ingredients into the dry and whisk them together, scraping the bowl down as needed. Pour in the hot coffee and continue mixing until smooth, but be careful not to overmix!
Pour the batter into the pan and bake for 35-45 minutes, until a toothpick inserted in the center comes out clean.
Let cake in the tin for about half an hour, then turn it upside down onto a cooling rack and leave to cool completely.