Stir sour cream and liqueur until smooth. Season to taste with sugar. Cut the biscuits in half except for 6 pieces. Divide into 6 dessert glasses or bowls
Whip the cream until stiff and allow the vanilla sugar to trickle in. Put 3 tablespoons of cream in a piping bag and fold the rest into the cream. Spread the sour cream on the sponge cakes. Chill for about 30 minutes. Spray cream tuffs onto the cream. Decorate with remaining lady fingers and pistachios