Place the condensed milk in a mixing bowl and whip for 2-3 minutes until creamy, then pour in 1 sachet of cream firming agent. Continue beating for approx. 1 minute. Whip the sour cream for approx. 4 minutes until it is firm, then add 1 sachet of cream firming agent. Carefully fold the sour cream into the "milk cream
Pour blueberries into a sieve and drain well. Rinse square springform pan (approx. 24 x 24 cm) briefly, then cover with foil. Line the bottom with 12 biscuits. Spread 1/4 of the cream on top, then sprinkle 1/4 of the blueberries on top. Layer 3 more layers of biscuits, cream and blueberries. Keep the slices cold for at least 6 hours, preferably overnight
Just before serving, lift the slices out of the mould and cut them into 16 pieces. Serve immediately, as the cream is very soft
waiting time approx. 6 hours