Method

• 1.
  For the double-choc ganache, place butter, both chocolates and sour cream in a large heatproof bowl. Set bowl over a small saucepan of gently simmering water (don’t let the bowl touch the water). Cook, stirring occasionally, until mixture is melted and smooth. Add icing sugar and whisk until very well combined and smooth. Strain into a medium heatproof bowl and cover surface directly with plastic wrap. Chill for 3 hours or until ganache is a thick, spreadable consistency that holds its shape.
• 2.
  To make the cake, preheat oven to 180°C/160°C fan-forced. Line base and sides of a deep 20cm square cake pan with baking paper.
• 3.
  Place brown sugar, butter, milk and cocoa powder in a large saucepan over medium-low heat and cook, stirring constantly, for 6-8 minutes until mixture has melted and is smooth. Remove pan from heat and stand at room temperature for 5 minutes to cool slightly. Add flour, baking powder and eggs to chocolate mixture and whisk vigorously until well combined and smooth. Pour into prepared cake pan and level surface.
• 4.
  Bake for 40-45 minutes until cooked when a skewer inserted in the centre comes out clean. Cool for 5 minutes in pan, then remove from pan and transfer to a wire rack to cool completely.
• 5.
  Transfer cooled cake to a serving plate. Remove double-choc ganache from fridge and stir until smooth, then thickly spread over top and sides of cake. Serve.