Pre-heat the oven to 160C fan-forced and grease and line a 30x20cm baking tin. Make sure you extend the paper over the sides.

Place the butter in a medium saucepan and heat it over medium head until it melts then add the water and cocoa and bring it almost to boiling, stirring constantly.

Place the flour, sugar, baking soda and salt in the bowl of an electric mixer and briefly beat them together.

Add the cocoa mixture to the flour mixture and beat on low speed until just combined then increase the speed to high until everything is well mixed in.

Add the eggs, buttermilk, vanilla and red food colouring and beat on low until everything is mixed in then up the speed to high for one minute.

Scrape the batter into the pan (it will be very runny).

Bake the cake for 25-30 minutes or until a cake tester inserted in the middle comes out clean.

Leave it to cool in the tin on a wire rack.

Meanwhile make the buttercream icing.

Place the sugar, milk, flour and salt in a medium saucepan and heat, whisking constantly, over medium heat until the mixture thickens and starts to bubble.

Reduce the heat to low and heat, continuing to whisk, for a minute.

Remove the pot from the heat and stir in the vanilla then leave the mixture to cool to room temperature.

Transfer the cooled mixture to the bowl of an electric mixer.

On medium speed, start to beat the mixture and then add the butter one tablespoon at a time.

Keep beating until it is thick and smooth (it may look curdled but keep beating and it will come together).

Spread the cooled cake with buttercream icing.

Serve immediately.