01 - In a medium saucepan, whisk together the whole milk, sweetened condensed milk, granulated sugar, and cornstarch until smooth.
02 - Place the saucepan over medium heat and cook, stirring constantly, until the mixture thickens and begins to bubble, approximately 6 to 8 minutes.
03 - Remove from heat and stir in the unsalted butter and vanilla extract until fully blended.
04 - Allow the custard to cool at room temperature, then refrigerate for at least one hour until chilled.
05 - Whip the heavy cream to stiff peaks using an electric mixer or whisk in a chilled bowl.
06 - Gently fold half of the whipped cream into the chilled custard to lighten the texture.
07 - Layer the custard mixture into serving glasses, alternating with dollops of the remaining whipped cream and fresh berries.
08 - Top with additional berries and optional chopped nuts. Serve chilled.