Preheat the oven to 160C.
In the bowl of a stand mixer combine the caster sugar and eggs and mix with the whisk attachment until the eggs and sugar have tripled in volume and are light and pale.
Line a large square baking tin (30cm x 30cm) with baking parchment.
Put the chocolate and butter in a bowl over a pan of simmering water, ensuring the water is not touching the base of the bowl. Stir occasionally until melted and combined together.
Weigh out the flour & cocoa into a stainless bowl. Once the butter and chocolate have melted add the dry ingredients to them. Mix thoroughly.
Fold in a third of the whisked eggs/sugar in to the bowl and combine. Add the remainder and gently fold through trying to lose as little volume as possible.
Pour in to the lined container and bake for 16-18 mins. You should see the top beginning to crack a little and the middle of the cake will still be gooey.
Fridge overnight and slice when cold.