01 - In a medium saucepan, whisk together the milk, sweetened condensed milk, sugar, and cornstarch until smooth.
02 - Place the saucepan over medium heat and cook, stirring constantly, until the mixture thickens and just begins to bubble, about 6–8 minutes.
03 - Remove from heat; stir in the butter and vanilla extract until fully incorporated.
04 - Let the custard cool to room temperature, then refrigerate for at least 1 hour until chilled.
05 - Whip the 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 it.
07 - Layer the creamy custard mixture into serving glasses, alternating with dollops of the remaining whipped cream and fresh berries.
08 - Top with additional berries and chopped nuts if desired. Serve chilled.