Melt 250g butter (diced) with 300g dark chocolate (cut up into pieces) in a heatproof bowl set over a pan of simmering water, stirring occasionally to combine. Set aside.

Whisk 3 eggs with 275g light brown sugar until the mixture has lightened and tripled in size (use an electric beater if possible).

Fold the chocolate mixture into the egg bowl, then add 75g cornstarch/cornflour (US/UK terminology) and 1tsp salt.

Fold in 100g walnuts, chopped (optional).

Pour into a lined and greased 24cm square baking tin and bake at 180c for 30-35 minutes (it should still be a bit squidgy in the middle when you take it out of the oven).

Let cool before cutting into squares (makes 20).