Preheat your oven to 350°F (175°C). Line a 9×9 inch baking dish with parchment paper, leaving an overhang on the sides for easy removal.

In a medium bowl, mix the graham cracker crumbs, melted butter, and granulated sugar until well combined. Press the mixture firmly into the bottom of the prepared baking dish. Bake for 10 minutes, then set aside to cool.

In a medium saucepan, heat the whole milk and heavy cream over medium heat until just simmering.

In a separate bowl, whisk together the granulated sugar, egg yolks, vanilla extract, and cornstarch until smooth.

Slowly pour a small amount of the hot milk mixture into the egg mixture, whisking constantly to prevent curdling. Gradually add the rest of the milk mixture while continuing to whisk.

Return the mixture to the saucepan and cook over medium heat, stirring constantly, until thickened and bubbling, about 5-7 minutes. Remove from heat and let cool slightly.

Pour the custard over the cooled crust and spread it into an even layer. Refrigerate for at least 2 hours, or until the custard is set.

In a large bowl, beat the heavy cream, powdered sugar, and vanilla extract until stiff peaks form. Spread the whipped cream over the set custard layer, creating an even, smooth top.

Cut into squares for serving and garnish with additional whipped cream or white chocolate shavings if desired.