Preheat the oven to 180C/160C. Grease and line a 30 x 20 cm rectangular tray with baking or parchment paper.
In a large mixing bowl, add flour, cocoa powder, sugars, baking powder and baking soda. Boil the kettle and pour out 1/2 cup of hot water and set aside.
In a separate bowl, add butter, eggs, vanilla and buttermilk and stir briefly.
Add butter mixture to dry ingredients and stir to combine. Add hot water and stir until chocolate cake batter is smooth.
Place in the oven for 45-50 minutes or until a skewer inserted in the middle comes out clean. Leave on a wire rack to cool completely.