Grease and line the base of a 23 cm (9 inch) springform tin. Mix the biscuit crumbs with the melted butter and press into the base of the prepared tin.
Melt the plain dark and white chocolate in separate bowls over pans of simmering water (see
Whiz the cream cheese, fromage frais, sugar and eggs in a food processor until smooth. Tip half the mix into the melted plain dark chocolate, and the other half into the melted white chocolate, and stir until smooth. Spoon half the white chocolate mix over the biscuit base, top with the dark chocolate mix, then finish with the rest of the white chocolate mix. Drag a skewer through the white and dark layers to marble the top.
Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 50 minutes until slightly risen and just firm to the touch. Leave to cool, then chill.