Line two medium-sized (roughly 20 x 30 cm – or equivalent) baking trays with baking paper and set aside.
Put the sugar in a large, heavy saucepan with 470ml (2 cups) of cold water and heat over a medium heat, stirring, until the sugar has melted (about 5 minutes). Then bring to the boil and boil (without stirring) until the mixture turns a deep caramel colour (you will need a bit of patience for this – it could take 20 minutes or so).
Remove from the heat immediately and quickly stir in the butter and then the peanuts. Immediately pour the mixture into the pans.
Melt the milk chocolate in the microwave or in a bowl over a pan of simmering water, then drizzle all over the peanut brittle. Repeat with the white chocolate. Set the trays aside and leave to cool completely for a few hours or even overnight before breaking into rough pieces.