Heat the oven to 180°C/fan 160°C, 350°F, Gas Mark 4.
Grease and baseline a 20cm square baking tin.
Place the butter and half the white chocolate in a bowl. Rest the bowl over a pan of gently simmering water. Do not let the bowl touch the water. Leave until the chocolate and butter is melted, stirring occasionally to mix. Remove the bowl from the pan and leave to cool slightly.
Whisk together the eggs, sugar and Nielsen-Massey Vanilla Extract. Add the chocolate and butter mix and whisk well.
Sift in the flour and salt, and then add the remaining chopped white chocolate and macadamia nuts. Pour the mixture into the pan. Bake for 20 – 25 minutes or until set and light golden/blonde brown. Leave to cool in the tin before cutting into squares.