Preheat the oven to 350F (180C gas 4 )
Butter and flour a 9x13 inch (33 x 22 cm) baking pan.
Line with baking paper leaving pieces to overhanging the sides.
Place the butter and dark chocolate in a double boiler and stir over barely simmering water until melted, remove from the heat and set aside to cool.
Beat the eggs and both sugars in a large bowl with an electric mixer at high speed until creamy, then with the mixer on low speed, beat in the salt and chocolate mixture, followed by the vanilla.
Gradually beat in the flour, stir in the milk chocolate and then spoon the batter into the prepared pan.
Bake until the brownies are set in the middle, 25-30 minutes. A toothpick inserted into the centre will come out slightly gooey.
Cool in the pan on a wire rack for 30 minutes. Then use the overhanging baking paper to lift the brownies out of the pan.
Remove the baking paper and place on a cutting board. Chill in the refrigerator for 15 mins before cutting.