Preheat the oven to 160 degrees celsius (fan-forced).
Grease the base and sides of a 20cm springform tin very well. Line the base with baking paper. Cover the outside of the tin with foil.
Place the springform tin into a baking tray that is large enough to fit the cake tin (sitting flat).
Insert the butterfly into the Thermomix bowl. Add the eggs and beat for 8 minutes on Speed 4, 37 degrees. Pour the eggs into a separate bowl and set aside.
Clean and dry the Thermomix bowl. Add the chocolate to the Thermomix and grate on Speed 8 for 10 seconds. Scrape down the sides of the bowl.
Add the chopped butter and melt for 5 minutes on Speed 3, 50 degrees.
Using a rubber spatula, gently fold ¼ of the egg mixture into the chocolate mixture. Repeat 3 more times with the remaining egg mixture.
When the mixture is completely combined, pour it into the prepared springform tin.
Boil water and carefully pour it into the large baking tray until it covers ⅔ of the sides of the springform pan.
Bake in the oven for approximately 30-35 minutes or until the chocolate cake begins to set on the outside edges (not in the middle). Remove the pan from the oven and set aside until the boiling water has cooled slightly.
When it’s safe to do so, carefully remove the springform tin from the water bath and set aside to cool completely.
Place the cake (still in the tin) into the fridge and leave until it's completely cold. For best results, leave in the fridge overnight. Allow to come to room temperature before serving.
Before serving, carefully remove the cake from the springform tin and discard and baking paper. Decorate with chocolate ganache, icing sugar, fresh berries, whipped cream or sifted cocoa powder or icing sugar.