01 - Whisk whole milk, sweetened condensed milk, sugar, and cornstarch in a medium saucepan until smooth.
02 - Heat mixture over medium heat, stirring constantly, until thickened and beginning to bubble, about 6 to 8 minutes.
03 - Remove saucepan from heat and stir in unsalted butter and vanilla extract until fully blended.
04 - Allow custard to cool to room temperature, then refrigerate for at least one hour until thoroughly 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 texture.
07 - Layer the custard mixture into serving glasses, alternating with spoonfuls of remaining whipped cream and fresh berries.
08 - Top with additional berries and optional chopped nuts; serve chilled.