Start by cooking blackberries with water over medium heat, mashing them gently. Simmer for a few minutes, then strain through a fine mesh to remove seeds. Return the juice to the pan, add sugar and lemon juice, and reduce slightly. Chill until room temperature.
Whip heavy cream in a chilled bowl until stiff peaks form, then set aside. Beat mascarpone with sugar, blackberry syrup, and vanilla. Gently fold in whipped cream to create a smooth, airy filling that’s bursting with berry flavor.
Dip ladyfingers into the cooled blackberry syrup and arrange them in the baking dish. Layer half the mascarpone filling over the soaked cookies, then repeat with another layer of syrup-soaked ladyfingers and the remaining filling. Optional: pipe the last portion for a polished look. Chill for at least 6 hours before serving.