Make sure you use pure maple syrup, not "maple-flavored" or "pancake" syrup. Martha made this recipe on episode 701 of Martha Bakes.
Make the crust: Roll out 1 disk of dough to just under 1/4 inch thick on a lightly floured surface and fit into a 9-inch glass pie plate. Trim excess, leaving a 1-inch overhang; fold edges under and set aside. Roll out half of remaining disk of dough (reserve remaining dough for another use) to just under 1/4 inch thick, cut out small leaf shapes, and mark veins with the back of a knife, if desired. Brush edges of piecrust with egg wash and apply leaves. Freeze for at least 1 hour.
Preheat oven to 375 degrees. Brush pastry leaves with more egg wash. Line crust with parchment and fill with pie weights or dried beans. Bake until crust is set, 25 to 30 minutes. Remove pie weights and parchment, and continue to bake until bottom of crust is lightly browned, about 10 minutes more. Let cool completely on a wire rack, about 1 hour.
Make the filling: Reduce oven temperature to 300 degrees. Place syrup in a large saucepan over medium-high and cook until reduced by a quarter, 5 to 7 minutes. Stir in cream and bring to a simmer; remove from heat.
In a medium bowl, whisk together egg, yolks, vanilla, salt, and nutmeg. Whisking constantly, slowly add cream mixture to egg mixture, and strain custard mixture through a fine-mesh sieve into a large glass measuring cup.
Pour custard into crust and place on a rimmed baking sheet. Bake until filling is just set but still slightly wobbly, 45 to 60 minutes. Let cool to room temperature before serving.