Whisk sugar, cornstarch & salt in a medium saucepan; whisk in milk.
Cook over medium high heat, stirring constantly, until bubbling and thick.
Temper yolks in a medium bowl and add to milk mixture.
Continue cooking another 2 minutes.
Remove from heat, add in cinnamon and vanilla.
Add butter, one tablespoon at a time, whisking until butter melts before adding another piece.
Let mixture cool 10 minutes. Gently fold in berries until evenly distributed.
Pour custard into graham cracker crust. Press plastic wrap directly on surface of custard.
Refrigerate until custard filling is chilled and firm, at least 4 hours.
Top fruit pie with whipped cream and more fresh berries. Serves 8.