In a large bowl, use an electric mixer to whip the cream, powdered sugar, vanilla and salt until soft peaks form. Set aside 3/4 cup of the whipped cream. Very gently, use a rubber spatula to fold in the strawberries to the cream in the bowl, being careful not to deflate the whipped cream.
Smear the 3/4 cup whipped cream on the bottom of a 13x9-inch dish and layer the graham crackers on top of the whipped cream so they are touching but not overlapping. Spoon about 1 cup of the berry whipped cream on top of the graham crackers and gently spread it evenly over them. Repeat this layered three more times, finishing with the whipped cream. You'll have four layers of graham crackers and four layers of whipped cream.
Use extra strawberries and blueberries to garnish the top of the cake.
Refrigerate the cake for at least 2 hours. The graham crackers will soften as the cake chills. Do not let it sit for more than 24 hours or it will get too soggy. Remove the cake from the refrigerator when you are ready to serve it. Cut into squares with a sharp knife, garnish with fresh mint (if using), and enjoy immediately.