Divide the diced strawberries (or raspberries) evenly among the 7 mason jars; set aside while you prepare the Jell-O.
Prepare the Jell-O according to the manufacturers directions on the back of the box. Pour it evenly among the 7 mason jars. Let the Jell-O set in the fridge for at least 8 hours or over night. (Can be made a day in advance.)
When ready to serve, remove from fridge and make the whipped cream.
Using a hand mixer, whip the heavy whipping cream in a medium size bowl until it's stiff but not over whipped. Add the powdered sugar and vanilla and stir to combine.
Add the whipped cream to the top of the Jell-O in each container using the back of a spoon or your finger to even the whipped cream out and get it all the way to the sides of the jar.
Garnish the whipped cream with fresh blueberries and blackberries.