250g unsalted butter
150g dark chocolate, chopped
300g caster sugar
150g plain flour
Preheat the oven to 160°C. Line a rectangular baking dish with baking paper.
In a double boiler or bain-marie, melt the butter with the dark chocolate and ¾ of the toblerone, stirring regularly. Allow to cool slightly.
In a large bowl, whisk the eggs with the caster sugar until light and fluffy. Fold the melted chocolate through the whisked eggs.
Sift the flour and cocoa into the bowl and fold through. Roughly chop the remaining toblerone and add it to the mixture.
Pour the mixture into the prepared dish and bake for 45 minutes. Allow to cool slightly before cutting into squares to serve.