Place a rimmed baking sheet on a middle rack in the oven and preheat to 350°F (180°C).
On a floured surface, roll out the buttermilk pie crust to an 1/8 -inch (3 mm) thick round and line a 9-inch (23 cm) pie pan with it. Crimp the edges, brush with egg wash and refrigerate while you prepare the filling.
In a medium saucepan over medium heat, bring the maple syrup to a vigorous simmer. Let it reduce for 5 minutes, adjusting the heat if necessary if it starts to bubble up too much.
Remove from the heat and stir in the cold butter until it has melted, then stir in the cold whipping cream. Let cool for 15 minutes.
Whisk in the eggs, followed by the flour and salt, until very smooth and free of any lumps.
Pour into your prepared crust and bake on the hot baking sheet for 50-60 minutes, or until it is nicely browned, a little puffed and still a bit jiggly in the center.
Let cool fully for at least 2 hours before serving. If you wish to serve this cold, transfer the cooled pie to the refrigerator for at least 4 hours to fully chill.
Store leftovers loosely covered in the refrigerator for up to 2 days.