Preheat the oven to 350°F (175°C). Line a 9×9-inch baking pan with parchment paper.

In a medium bowl, combine graham cracker crumbs, melted butter, and 1/4 cup granulated sugar. Press the mixture firmly into the bottom of the prepared pan to form the crust. Bake for 10 minutes, then let it cool completely.

In a medium saucepan, heat the milk and heavy cream over medium heat until it begins to simmer.

In a separate bowl, whisk together the sugar and flour. Gradually add the egg yolks, whisking until smooth.

Slowly pour the hot milk mixture into the egg mixture, whisking constantly to prevent curdling. Return the mixture to the saucepan and cook over medium heat, stirring constantly, until it thickens and comes to a boil.

Remove from heat and stir in the vanilla extract.

Pour the custard over the cooled crust and spread it evenly.

Refrigerate for at least 4 hours, or until set.

Cut into squares and dust with powdered sugar before serving.