Preheat the oven to
180°C/350°F/Gas mark 4. Lightly grease a 20cm/8in square
tin and line with baking parchment, leaving the paper
hanging over on 2 opposite sides.
Place the butter and
white chocolate in a heatproof bowl. Half fill a
saucepan with water and bring to the boil. Remove from
the heat.
Place the bowl over the saucepan, making sure
the bottom of the bowl does not sit in the water. Stir
occasionally until the butter and chocolate have melted
and are smooth.
Add the caster sugar to the bowl and
gradually stir in the eggs. Add the vanilla essence,
fold in the flour and macadamia nuts, then pour into the
prepared tin.
Bake for 35 to 40 minutes. Leave to cool
in the tin before lifting out and cutting into squares.