• 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.

Buttercream

• Place butter in a large mixing bowl and beat with an electric mixer on medium speed until pale and creamy.
• Add half the sugar and one tablespoon of milk and beat until combined.
• Add the remaining milk and the mint extract and beat until smooth.