Method
Preheat the oven to 170°C. Line the tin with baking paper so it comes up all the sides.
Melt the chocolate and butter in heatproof bowl, set over a pan of gently simmering water, making sure the bottom of the bowl is not in contact with the water. Stir constantly. Set aside to cool slightly.
Whisk the eggs and caster sugar in a large mixing bowl until light and creamy.
Fold in the slightly cooled melted chocolate and butter.
Spoon the mixture into the prepared cake tin and bake in the preheated oven for 15 minutes, until the centre is just firm to the touch.
Remove from oven, lift out of the tin using the baking paper and allow to cool a little before slicing.