Put the chocolate and butter in a heavy-based saucepan over a very low heat and stir until both ingredients are just melted. Remove from the heat, stir in the sugar and set aside to cool a little.
Pour the chocolate mixture into a large bowl and beat in the eggs, orange rind and orange essence (if used).
Sift the flour, salt and baking powder into the bowl and fold in, together with the chocolate chunks and macadamia nuts.
Pour the mixture into a greased and lined cake tin, about 20 x 30 x 5 cm (8 x 12 x 2 inches).
Cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 25–30 minutes or until set but not too firm. Leave the brownie to cool in the tin, then cut it into squares and serve.