In a medium mixing bowl add the maple sugar, cornstarch, and salt. Whisk together to combine.
In a room temperature large sauce pan whisk together the eggs, egg yolks, heavy cream, vanilla, and maple syrup until smooth.
Strain the egg mixture through a fine mesh strainer into the medium mixing bowl with the dry ingredients.
Whisk the dry and wet ingredients together until smooth. Strain the mixture back into the large sauce pan and stir in the buttermilk.
Preheat the oven to 350℉
Heat the large sauce pan over medium low heat to pre-cook the pie filling. DO NOT BOIL. Stirring constantly with a rubber spatula scraping the bottom of the sauce pan cook the custard for 5-10 minutes until it starts to thicken. If you're extra precise, cook to a temperature of approximately 160℉.
Carefully pour the thickened pie filling into the prepared graham cracker pie crust.
Bake the pie on the center rack for 30-40 minutes. The edges of the pie should look puffed and set, but the center of the pie should jiggle. If you're extra precise, the temperature of your pie should be between 170-175℉.
Allow to cool on the counter.
While the pie is cooling on the counter, make the maple whipped cream by combining all ingredients and whipping to soft/medium peaks.
Once the pie has come to room temperature, top with the maple whipped cream. Chill for at least 3 hours before slicing and serving.
Optional: finish the top of the pie with a dusting of maple sugar immediately before serving.