Preheat the oven to 180°C and grease a 20cm square cake tin with butter.
Beat sugar and eggs until pale and fluffy, then gradually incorporate the melted butter.
Carefully fold the sifted flour into the egg mixture using a metal spoon.
Fold in the chopped white chocolate and macadamia nuts.
Spread the batter into the tin and bake for 30-35 minutes until a skewer comes out clean.
Whisk butter, icing sugar, and vanilla seeds together until smooth.
Add peanut butter and whisk until smooth, adding milk if needed to loosen the frosting.
Spread the frosting over the cooled blondies and cut into squares to serve.