For the Crust:

1. Preheat the oven to 350°F.
2. Combine the vanilla wafers, sugar, and salt in the bowl of a food processor. Process until fine crumbs form.
3. With the machine running, slowly stream in the melted butter. Pulse a few times until all the crumbs are moistened.
4. Press the mixture into the bottom and up the sides of a 9-inch tart or pie dish.
5. Bake for 7-8 minutes or until lightly browned. Cool completely.

For the Filling:

1. While the crust is baking, make the filling by combining the sugar, flour, and salt in a small saucepan. Place the pan over medium heat and slowly stream in the milk, whisking well to incorporate.
2. Cook, stirring constantly, until the mixture bubbles and thickens. Reduce the heat to low and continue cooking for another couple minutes.
3. Whisk the egg yolks together in a bowl. Carefully ladle some of the hot milk mixture into the egg yolks and whisk them well to incorporate (this tempers the egg yolks and prevents them from scrambling). Pour the egg mixture into the pot with the milk mixture, whisking well.
4. Bring the mixture to a simmer, stirring constantly, and cook for 2-3 minutes.
5. Remove the pan from the heat and whisk in the vanilla and espresso powder. Pour the mixture into the prepared crust and allow it to cool to room temperature. Place in the fridge until set, at least 3 hours.

For the Topping:

1. Combine the heavy cream and sugar in the bowl of a mixer. Beat until the cream is thickened and soft peaks form. Gently stir in the vanilla.
2. If desired, remove the chilled tart from the pan. Top the tart with the whipped cream and garnish with shaved chocolate. Slice and serve.