Prepare the pastry in a food processor, combine the walnuts and sugar and pulse until the nuts are finely ground. Add the flour and salt and pulse to incorporate. Add the butter and pulse until the mixture is the texture of coarse meal with pea size pieces. Add 2 tablespoons of ice water and pulse until the dough is evenly moistened. The dough should hold together when squeezed, but not be too wet. Add up to 2 more tablespoons of ice water, if necessary. Tip out the dough onto a piece of plastic wrap and using the plastic to gather the dough together, form it into a disk. Chill for at least 2 hours or up to 2 days. (Alternatively, you can freeze the dough for up to 1 week).
Preheat oven to 375 degrees F. On a lightly floured surface, roll out the dough into a 12-inch-diameter round (about 1/8 inch thick). Transfer the dough to a 9-inch standard pie plate. Trim the excess dough and fold it to make a decorative edge. Wrap in plastic wrap and freeze for at least 15 minutes. Line the frozen shell with parchment paper or foil and fill with pie weights. Bake the crust until the edges are lightly golden and the crust beneath the parchment is dry, about 25 minutes. Remove the parchment and weights and continue to bake the crust until it is golden brown about 15 to 20 minutes. Transfer the pan to a wire rack to cool completely. Set a fine mesh sieve next to the crust for later.
Prepare the filling: In a small saucepan, bring the maple syrup to a boil over high heat, then lower the heat to maintain a gentle simmer. Continue to cook the syrup until it has reduced by about half, about 12 minutes. (You can occasionally pour the syrup into a glass measuring cup to check the volume.) In a medium saucepan (away from the stovetop), whisk the egg yolks with the cornstarch until smooth. In a slow stream, while whisking, add the milk and cream. Add the salt, butter, and reduced maple syrup. Don't worry if the syrup seizes. It will smooth out in the next step when you heat the custard. Cook the milk mixture over medium heat, stirring constantly with a wooden spoon, until it has thickened and come to a low boil, about 7 minutes. Cook the custard for 1 minute more and then pour it through the sieve into the cooled crust and spread it out evenly.
Press a piece of waxed paper directly onto the surface of the custard and transfer the pie to the fridge. Chill the pie for at least 4 hours or up to overnight. To serve, top the pie with whipped cream.