Preheat the oven to 180C/355F/Gas 4.
Line the sides and base of a 33cm x 23cm x 5.5cm/13in x 9in x 2in baking tin with aluminium foil and lightly grease with butter.
Place the butter and chocolate into a heatproof bowl. Place the bowl over a saucepan of barely simmering water (do not let the base of the bowl touch the simmering water) and allow to melt.
Place the eggs, sugar and vanilla extract into a clean bowl and whisk together well.
Once the chocolate and butter have melted, set aside and allow to cool slightly.
Add the egg mixture and whisk to combine.
Sift the flour and salt together then fold into the chocolate mixture.
Add the white chocolate pieces and mix to combine.
Pour the mixture into the baking tin and place into the oven to bake for 25 minutes. Remove from the oven and allow to cool in the tin.
To serve, remove from the baking tin and cut the brownie into squares while still warm. Pile up the brownie squares onto a large plate and sprinkle with icing sugar.