Line a flat plate with baking paper. Set aside. The plate should be small enough to fit in the fridge.
In a food processor, blend together the cream cheese, mascaropone, and salt until smooth.
Transfer to a medium bowl and put into the fridge for half an hour for it to harden.
Chop the pistachio nuts and pour it in a small bowl.
After the cheese is a bit harder, form little balls with your hands with a grape in each. The depth of the cheese should be 5mm on the grapes. Place the prepared balls on the plate into the fridge right after they are ready, one by one.
When all the balls are ready, using your hands, roll the balls to the Pistachio mixture.