Preheat oven to 160°C. Lightly grease a 27cm x 18cm shallow tin and line the base and two sides with baking paper.
Melt the butter and stir in the cocoa then add the eggs one at a time, beating well after each addition.
Add the sugar, vanilla, and sifted flour and baking powder to the mixture and stir until well combined. Then stir through the chopped chocolate and pour the mixture into the prepared tin.
Bake for 45-50 minutes or until the mixture is just firm when pressed in the centre. Don’t over-bake or the brownies will not have the desired fudgy centre. Leave in the tin for 20 minutes before turning out onto a wire rack to cool. Remove the baking paper and reverse onto another rack.
Dust with icing sugar and cut into squares when cold.