Preheat oven to 160C/150fan/Gas3
Line the cake tin with greaseproof paper
Melt the chocolate and butter together in a bowl suspended over a pan of simmering water (don’t let it touch the water), or in a microwave using 30 second bursts.
Using a stand or hand mixer, whisk eggs with 90g sugar until they triple in size
Heat the remaining sugar in a saucepan with 30g water. Use the thermometer to bring to 121C then remove from the heat.
Slowly and carefully pour the sugar syrup over the egg mixture whilst whipping slowly
Fold the egg and sugar mixture into the melted chocolate and butter mixture. When combined, pour into the lined cake tin.
Place the tin into the baking tray and fill the tray with warm water until it comes threequarters of the way up the side of the tin.
Place the tin in the oven for one hour
Allow to cool at room temperature overnight
Turn out of the tin & serve in slices with whipped cream.