Drain the cherries well and collect the juice. Mix the starch with approx. 2 tbsp. cherry juice until smooth. Boil up the rest of the juice. Stir in the starch. Simmer while stirring for about 1 minute, remove from heat. Let the cherry juice cool down.
In the meantime, lightly oil a tray. If necessary, make a ring (approx. 8 cm Ø and 6 cm high) from aluminium foil that has been folded several times.
For the tartes, melt the butter and let it cool down a little. Place the chocolate cookie bars in the freezer bag, close the bag and roll over with a pastry roll until the cookies are finely crumbled.
Mix well with the liquid butter.
Place the ring on the tray. Pour in 1/6 (1-2 tbsp.) of the cookie crumb mixture each and press down lightly using the back of a tablespoon as a base. Carefully pull up the ring. Make 5 more bases from the remaining crumb mass.
Cover and chill for about 30 minutes.
Mash the cherries a little with a fork. Mix mascarpone and sugar briefly. Whip cream until stiff. Add the cherries to the mascarpone. Spread loosely on the biscuit bases. Cover and chill for about 1 hour.
Sprinkle the tartes with the cherry sauce and serve.