Rinse a square casserole dish (approx. 22 x 22 cm) cold and line with cling film. Drain blueberries well in a sieve.
For the cream, whisk the condensed milk with the whisk of the mixer for approx. 3 minutes until creamy. Add 1 cream stiffener and continue beating for approx. 1 minute. Whip the sour cream for approx. 4 minutes until firm.
Also pour in 1 cream stiffener, continue beating for approx. 1 minute. Stir the sour cream into the condensed milk cream.
Line the bottom of the mould with approx. 12 biscuits, breaking them to size if necessary. 1⁄4 Spread cream on them. 1⁄4 Spread berries on top. Also lay out 3 more layers of biscuits, cream and berries.
Cover and chill for at least 6 hours, preferably overnight.
Shortly before serving, lift the layer cake out of the mould using the foil. Open the foil and cut the cake into about 16 pieces. Serve immediately as the cream is quite soft.