Mix the berries, sugar and lemon juice in a pan, bring to the boil, simmer over a low heat for approx. 5 mins. Leave the compote to cool.
Blitz the cookies in a food processor, mix with the cocoa powder and butter. Place a cookie cutter on each plate, divide the mixture between the cookie cutters, press down firmly with a glass. Combine the mascarpone, sugar, lemon zest and lemon juice, spread on top of the biscuit bases. Top with half of the berry compote, set the rest aside. Cover the tarts and chill for approx. 2 hrs.
Carefully loosen the tarts from the cutters using the tip of a knife, remove the cutters. Serve with the reserved berry compote and blueberries.