Add the blackberries to a bowl, sprinkle over the granulated sugar and blackberry liqueur and let steep 10 minutes.
Meanwhile, add the cream and powdered sugar to the bowl of a mixer and whip to soft peaks. Mash the blackberries with a fork, and then fold them into the cream.
Spoon the fool into individual glasses. Crush some ladyfingers on top. Top each with a sprig of mint and a whole blackberry!