Preheat the oven to 180°C (fan 160°C) / 350°F / gas mark 4. Grease a 19cm (7½in) square shallow baking tin and line the base with baking paper. Melt the chopped chocolate, butter and vanilla together in a heatproof bowl set over a saucepan of barely simmering water, making sure the surface of the water does not touch the bowl. Remove from the heat and stir in the sugar, then leave to cool for a few minutes.
Beat in the eggs, then sift in the flour, cocoa and salt and fold in until the mixture is smooth and glossy. Stir in the chocolate chips. Pour the mixture into the prepared tin and level the top.
Bake in the oven for 25 minutes, or until the top starts to crack but the centre remains gooey. Turn off the oven and leave the brownies inside for a further 5 minutes before removing. Leave to cool completely in the tin.
Cut the brownies into 16 small squares and remove from the tin. Store in an airtight container for up to 4 days.
Recipe courtesy of Chocolat by Eric Lanlard, published by Mitchell Beazley with photography by Kate Whitaker.