Preheat your oven to 350°F (175°C). Process graham crackers into fine crumbs. Combine crumbs with brown sugar, sea salt, and melted butter, then press into the tart pan. Bake for 10 minutes until golden brown and cool completely.

Heat whole milk in a saucepan over medium heat until simmering. Whisk together egg yolks, granulated sugar, kosher salt, and cornstarch in a bowl. Gradually pour hot milk into the egg mixture while whisking, then return to the saucepan. Cook over medium-low heat for 5-7 minutes until thickened.

Strain custard through a fine-mesh sieve into a bowl. Add unsalted butter and stir until melted. Cool at room temperature for 30 minutes, cover with plastic wrap, and refrigerate until chilled, about 1 hour.

Remove plastic wrap from chilled custard and whip on medium speed until light. Pour whipped custard into cooled crust and smooth the top. Chill assembled tart in the refrigerator for at least 3 hours to set.

Prepare fresh fruit for topping, arrange it artistically on the tart just before serving, slice, and serve chilled.