- In a saucepan, cook the milk, egg yolks, sugar, and cornstarch over medium heat, stirring constantly, until mixture boils and thickens. Stir in vanilla and butter. Transfer custard to a shallow dish to cool.
- Meanwhile, in a saucepan, mix 1 cup of blueberries with sugar over medium to high heat. Stir occasionally, until the blueberries start to soften and burst and become a slurry. Mix the cornstarch into the water, then pour into the saucepan. Bring to a boil while continuing to stir, until the mixture thickens. Stir in lemon juice and butter, then remove from heat and let cool. Gently stir in remaining 3 cups of berries.
- Once the custard is near room temperature, spread it into the bottom of the graham cracker crust. Top with the berry mixture and refrigerate until set.