In a large saucepan, combine butter, sugars, cocoa, bicarb soda and water over low heat, stirring, until the sugar has dissolved and the butter has melted.
Bring mixture to the boil and simmer for 5 minutes.
Take mixture off heat and allow to cool to room temperature.
While the mixture is cooling, pre-heat the oven to moderate (180 degrees celsius/350 degrees fahrenheit, fan-forced).
Grease a round cake tin (19cm/7.5 inch) and line with baking paper.
Once boiled mixture has cooled to room temperature, stir in the eggs and flour until combined.
Pour mixture into cake tin and bake for 40-45 minutes.
Allow to cool in tin for 10 minutes before turning out onto a cooling rack.
Allow to cool completely before icing.
Chocolate Icing
Sift icing sugar and cocoa into a bowl. Make a well and place butter in the centre.
Add boiling water in middle of well and wait 30 seconds for the butter to mostly melt. Stir to combine.
Pour icing over cake and use a spatula to smooth around the edges.
Notes
TIPS:
This cake requires about an hour to allow the boiled mixture to cool, so start by boiling the necessary ingredients and once it's cooling you can start the usual cake baking prep.
Use a much larger saucepan than you think you'll need as the boiled mixture froths up to many times its original volume.
When waiting for boiled mixture to cool, you can place it into the fridge to speed up the cooling process – but don't let it get cold, you're aiming for room temperature.
Use a spatula or wooden spoon to combine the flour into the mixture, however, if you have trouble getting rid of lumps, stir with a whisk until the mixture is smooth.
You will know the cake is cooked when the middle doesn't wobble as you attempt to remove it from the oven and the middle feels firm to touch.
If your cake sinks a little in the middle when you take it out, don't cry. This cake is more resilient and forgiving than a sponge. Turn it out upside down onto a cooling tray after a few minutes (rather than waiting 10 minutes) and leave it upside down to cool completely. It should pretty much pop back into shape.
STORAGE:
It will last for up to five days stored in an airtight container – in or out the fridge.
Wrap the cake well in food wrap and it will be fine frozen for up to three months.