Preheat your oven to 375°F (190°C).

Place the unbaked pie crust into a 9-inch pie dish and crimp the edges desired. Keep it chilled in the refrigerator while you prepare the filling.

In a large mixing bowl, whisk together the brown sugar and all-purpose flour until well combined and free of large lumps.

Gradually pour in the heavy cream, pure maple syrup, and vanilla extract, whisking constantly until the mixture is smooth and uniform.

Remove the pie crust from the refrigerator and scatter the chopped walnuts evenly over the bottom.

Carefully pour the liquid filling mixture over the walnuts.

Dot the top of the filling with the small cubes of unsalted butter.

Bake in the preheated oven for 40 to 45 minutes. The center should still be slightly jiggly like gelatin, as it will set further while cooling.

Remove from the oven and place on a wire rack to cool completely to room temperature before slicing. This allows the filling to firm up properly.