Melt the butter with the chocolate in a pan over a low heat, then leave to cool. You don't want the mixture to set, so make sure it doesn't go completely cold.
150 g unsalted butter, 225 g dark chocolate
Whisk the sugar with the eggs, vanilla extract and salt using electric beaters until the mixture is thick, pale and at least doubled in volume.
125 g dark brown sugar, 3 large eggs, 1 tsp vanilla extract, ¼ tsp fleur de sel
Sifted in the flour and fold in as lightly as possible. Lightly stir in 60g of the walnuts.
50 g buckwheat flour, 80 g walnuts
Fold in the chocolate mixture until just incorporated. Pour into a 22cm (9") sq silicon cake mould or lined tin and scatter the remaining 20g of walnuts over the top.
Bake at 180℃ (350℉, Gas 4) for about 20 minutes when the mixture should be well risen, but with a slight wobble in the middle. Leave to go cold then cut into 16 squares.
Notes
Please note: calories and other nutritional information are per serving. They're approximate and will depend on serving size and exact ingredients used.