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 stir constantly until the mixture thickens and begins to bubble, about 6 to 8 minutes.
03 - Remove from heat and stir in unsalted butter and vanilla extract until fully combined.
04 - Allow the custard to cool to room temperature, then refrigerate for at least 1 hour until chilled.
05 - In a chilled bowl, whip the heavy cream to stiff peaks.
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 mixed berries.
08 - Top with additional fresh berries and chopped nuts if desired. Serve chilled.