Put 250g of the plain chocolate in a mixing bowl. Bring the cream to the boil in a small pan, then pour over the chocolate. Add the liqueur, if using. Stir until the mixture is smooth and the chocolate has melted.
Set aside to cool and cover with cling film. Once your mixture has cooled chill it in the fridge for 2 hours or until firm. Remove and leave to soften at room temperature for 30 minutes, then roll the chocolate cream into balls with a teaspoon.
Stand a cooling rack over a baking sheet. In another pan melt the remaining 200g of plain chocolate. Use a fork to dip and coat the truffles in the melted dark chocolate, gently shaking off any excess. Place truffles on a cooling rack over a baking sheet.
Once set, melt the white chocolate in a bowl set in a pan over simmering water, or in small bursts in the microwave. Drizzle the melted white chocolate over in a zigzag pattern. Leave again to set. Transfer to petit-four cases.
Cook's tip: make ahead of your party, the truffles will keep for 1 week in the fridge.