• Preheat oven to 175C (350F) and line a 9 inch x 13 inch baking tray, leaving an ‘overhang’ of baking paper (this makes it easier to pull out after baking).

To make base

• In a medium bowl, whisk together flour and cocoa and icing sugar. Using your finger tips, rub in the butter until combined and crumbly.
• Press mixture into prepared baking tray and bake for 20 minutes.

To make topping

• While base is baking, prepare topping.
• In a medium sized bowl, sift sugar, cocoa and baking powder. Add slightly beaten eggs and sour cream and mix well, making sure all ‘streaks’ of egg and sour cream have gone.
• Pour on top of warm baked base and bake for 20-25 minutes.
• Cool completely before cutting into squares and dusting with cocoa powder.