You will need a 1lb loaf tin, cling film, three large bowls and a freezer.
60g white chocolate
120g dark chocolate
3 eggs, separated
200ml double cream
Make a sugar syrup: put the sugar in a small pan with a splash of water, and cook very gently until all of the sugar is dissolved. Boil for 2-3mins until thickened and lightly coloured. Allow to cool slightly.
Separate the eggs. In one bowl, whisk the yolks with half the sugar syrup until pale and frothy. In another bowl, whisk the whites to soft peaks and then slowly pour in the rest of the sugar syrup, whisking all the time, until glossy and thick. In the third bowl, whip the cream to soft peaks.
Melt the white chocolate in a bain marie or the microwave. Add 1/3 of the yolk mixture, then fold in 1/3 of the cream and 1/3 of the egg whites. Leave to cool in the fridge, then pipe or spoon onto the clingfilm. Wrap tightly into a sausage shape and leave to set in the freezer for half an hour.
Melt the dark chocolate in a bain marie or the microwave. Add the remaining yolk mixture, then fold in the remaining cream and egg whites. Pour half of the mixture into a loaf tin lined with clingfilm. Take the white chocolate from the freezer, unwrap and lay carefully in the middle of the loaf tin. It should be soft, but set enough to hold its shape.
Carefully pour the rest of the dark chocolate mixture around the sides and over the top of the white chocolate parfait, and loosely cover with cling film. Freeze the whole thing for eight hours, or over night.
To serve, turn the parfait out onto a plate, and cut into 1.5cm slices with a hot knife. I served mine with caramelised almonds, blackberries and a hot blackberry-brandy sauce.