Preheat oven to 180ºC / gas mark 4. Line the tin with baking parchment.
Melt the butter and chocolate together in a heavy-based pan over a low heat.
Beat eggs with the sugar and vanilla in a mixing bowl.
Allow the chocolate mixture to cool a little then whisk it into the eggs and sugar.
Sieve the flour into the mixture and add the nuts. Mix well.
Bake for 25 minutes or until it’s set with a slight wobble in the middle. Allow to cool and refrigerate for 20 minutes before cutting.