Place the chocolate in a thin-sided bowl and suspend over the gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate and stir until smooth. Beat the butter with a hand mixer for approx. 2 mins., stir in the sugar and liqueur. Stir in the chocolate and fold in the whipping cream. Cover 1/4 of the mixture and set aside at room temperature.
Fill the prepared tin to a depth of approx. 1 cm with a little of the chocolate mixture. Cover with approx. 4 Petit Beurre biscuits, divide a little of the chocolate mixture on top and continue until all the Petit Beurre biscuits are used up, finishing with the chocolate mixture. Cover and refrigerate for approx. 2 hrs. Remove the cake from the tin, coat with the reserved chocolate mixture, return to the fridge for about 30 mins. Dust with cocoa powder.