Line a baking tin, approximately 24 cm (9 inch) in diameter with plastic wrap allowing the excess to hang over the rim. Melt the butter. Crush the biscuits in a food processor or mortar. Mix the butter with the biscuit crumbs and press the mixture evenly on the bottom of the tin. Melt the chocolate in the microwave or over a bain-marie. Leave to slightly cool. Beat the cream and sugar until stiff. Carefully stir the melted chocolate into the whipped cream and add the liqueur. Fill the tin with the chocolate mixture and smooth out the top with a spatula. Cover the cake with the excess plastic wrap and place in the refrigerator to chill for at least 4 hours Before serving lift the cake out of the tin, remove the plastic wrap and sprinkle with cocoa powder. Serve small wedges with crème anglaise with lemongrass.