Step 1

Heat the oven to 300%deg;F

Step 2

Using extra butter, grease a 10-inch round cake pan and line with baking parchment.

Step 3

Break the chocolate into pieces and melt with butter in a bowl over simmering water. Beat the eggs and 5 tablespoons of the sugar in an electric mixer until the volume quadruples.

Step 4

Heat the remaining sugar with 7 tablespoons of water until dissolved into a light syrup. Pour the hot syrup into the melted chocolate and cool slightly.

Step 5

Add the chocolate to the eggs and beat slowly until the mixture is combined. Pour into the pan.

Step 6

Put a folded dish towel in the bottom of a roasting pan. Put in the cake and add enough hot water to come three-quarters of the way up the side of the cake pan.

Step 7

Bake in the oven for 1 hour until set. Leave the cake to cool in the water before unmolding.