Preheat oven to 350℉ (177℃).
Pulse the graham crackers in a food processor or crush in a ziploc bag until they are fine crumbs. Add the sugar and butter, then mix until evenly moistened. Press into the bottom and sides of a 9-inch pie pan. Bake for 10 minutes to set the crust.
1 1/2 cups graham cracker crumbs, 1/3 cup granulated sugar, 6 Tablespoons salted butter
Meanwhile, in a small saucepan over medium-high heat, bring the maple syrup to a boil, then reduce the heat to medium-low or low to maintain a gentle simmer. Continue to cook the syrup until it has reduced by half to about 2/3 cup (around 10-15 minutes). It is okay to occasionally pour the syrup into a heat-safe glass measuring cup to see how much more it needs to be reduced. Set aside to cool slightly.
1 ¼ cups pure maple syrup
In a medium pan, combine the egg yolks and cornstarch. Whisk well. Add the milk and cream, whisking well until combined. Transfer to the stovetop over medium heat and whisk in the salt, butter, the reduced maple syrup, and maple extract, if using. The syrup will likely seize and clump up, but just continue to cook over medium heat, stirring constantly, until the mixture has thickened and starts to bubble at a low boil, around 7 minutes. Cook for 1 minute more at a low boil, then pour the custard through a fine-mesh sieve to remove any cooked egg bits.
4 large egg yolks, ¼ cup cornstarch, 2 cups whole milk, 1 cup heavy cream, ½ teaspoon kosher salt, 2 Tablespoons butter, ½ teaspoon maple extract
Pour the custard into the prepared pie crust and spread in an even layer. Transfer to the fridge to chill for at least 3-4 hours.
When ready to serve, beat the heavy cream, powdered sugar, and vanilla in a bowl until medium-stiff peaks form. Spread over the maple custard layer, then slice and serve.
3 Tablespoons powdered sugar, pure vanilla extract, 1 cup heavy cream