225g unsalted butter, diced
100g dark chocolate, broken up
200g caster sugar
4 large free-range eggs, at room temperature
1⁄2 teaspoon vanilla extract
50g plain flour
50g cocoa powder
a good pinch of salt
75g walnut pieces icing sugar, for dusting a 20.5cm square tin, greased with butter and the base lined with greaseproof paper
Can be cut into 16
Heat the oven to 180°C/350°F/gas 4.
Put the butter and chocolate into a small, heavy-based pan. Set over the lowest possible heat and leave to melt gently, stirring frequently, until smooth but not hot. Remove the pan from the heat and leave to one side until needed.
Put the sugar, eggs and vanilla extract into a mixing bowl large enough to hold all the ingredients. Mix thoroughly with a wire whisk or wooden spoon. Whisk in the melted chocolate mixture. Sift the flour, cocoa and salt into the bowl and mix thoroughly. Stir in the nuts, then transfer the mixture to the prepared tin and spread evenly. Bang the tin on a work surface to expel any air bubbles.
Bake in the heated oven for about 25 minutes, until a skewer inserted into the centre of the mixture comes out with moist crumbs – the mixture will continue cooking after it comes out of the oven and it’s vital not to overcook it. Note that the mixture will puff up in the oven but sink on cooling.
Stand the tin on a wire cooling rack and leave to cool completely before cutting into squares. Dust with icing sugar just before serving. Store in an airtight container and eat within a week.