- With the rack in the middle position, preheat the oven to 350°F (180°C). Generously butter and flour (see note) a 10 to 12-cup (2.5-3 litre) Bundt pan.
- In a bowl, combine the flour, baking powder, baking soda and salt. Set aside.
- In another large bowl, whisk the eggs with the sugar, ¾ cup of the buttermilk, the oil and vanilla. Set aside.
- In a small bowl, whisk the cocoa powder with the remaining buttermilk (¾ cup). Add to the egg mixture. Add the dry ingredients and stir until smooth. Spread the batter into the pan.
- Bake for 45 to 50 minutes or until a toothpick inserted in the centre of the cake comes out clean. Let cool for 15 minutes. Unmould and let cool completely on a wire rack.
For a uniform, dark exterior, dust the pan with cocoa powder instead of flour.