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