Line the bottom of a charlotte mould or a deep 18cm/7in round cake tin with a piece of baking parchment.
Put the boudoir biscuits on a non-reactive tray, and sprinkle with half of the orange-flavoured liqueur. Use to line the sides of the mould or tin, trimming if necessary to make a tight fit.
Mix the chocolate with the double cream in a bowl, and beat in the egg yolks. Whisk the egg whites in a large bowl until standing in stiff peaks, then gradually add the caster sugar, whisking until stiff and glossy.
Carefully fold the egg whites into the chocolate mixture in two batches, taking care not to knock out the air. Pour into the centre of the mould, and trim the biscuits so they are level with the chocolate mixture. Chill for at least 5 hours before serving.