Mix the sour cream and liqueur until smooth. Add sugar to taste. Halve the biscuits, except for 6 pieces. Distribute in 6 dessert glasses or bowls.
Whip the cream until stiff while drizzling in the vanillin sugar. Put 3 tablespoons of cream in a piping bag and fold the rest into the cream. Spread the sour cream on the biscuits. Chill for about 30 minutes. Sprinkle cream tuffs on top of the cream. Garnish with the remaining ladyfingers and pistachios.