Grease and line a round springform cake tin. In a large mixing bowl, sift the flour, cocoa powder, baking soda, baking powder and then add the sugars. Pop the butter into the microwave for short bursts, 10 seconds or so at a time, until it is just melted. Give it a stir with a fork to eliminate any lumps. Whisk together the butter, eggs, vanilla and buttermilk until combined - don't worry if it goes a little lumpy. Then add the wet mixture into the dry mixture and gently fold until just combined.