Preheat oven to 160 degrees Celsius (145 degrees Celsius fan-forced).

Grease a 20cm square cake pan and line the base and sides of the pan with baking paper.

Place chocolate, butter, milk and sugar in a large saucepan over low heat, stirring frequently. Remove from heat when chocolate and butter have melted, and stir mixture until completely smooth. Allow mixture to cool at room temperature for 15 minutes.

Add vanilla and eggs to chocolate mixture and stir until well combined.

Stir flours together in a large bowl. Add one cup of chocolate mixture to the flour and stir until a smooth paste forms. Repeat with another cup of the chocolate mixture. Add remaining chocolate mixture and stir until mixture is smooth.

Pour mixture into prepared pan.

Bake for about 1 hour 10 minutes to 1 hour 20 minutes. When the cake is ready, a fine-bladed knife inserted into the centre of the cake should come out without any batter attached.

Loosely cover cake with greaseproof paper or a clean tea towel and allow it to cool to room temperature in pan.

The cooled cake can be iced with the white chocolate ganache immediately (see directions below), or stored and then iced on the day of serving.

Store cake in an airtight container in the refrigerator and bring to room temperature before serving.

Melt white chocolate in a small saucepan over very low heat, stirring frequently. When chocolate has completely melted, remove from heat and quickly stir in sour cream. Use immediately, or if your ganache is quite runny, set it aside at room temperature to thicken slightly before using.