Make the dough
Pulse the flour, sugar, and salt in a food processor once or twice to combine. Add the butter and pulse until the mixture is pebbly, with some pea-size bits of butter.
In a small bowl, whisk together the egg yolk, lemon juice, and 3 Tbs. of the ice water. Pour the mixture through the feed tube of the food processor, pulsing to combine. The dough mixture should be moist but should not come together into a ball. If a small handful of dough pressed together does not adhere, add the remaining 1 Tbs. ice water, pulsing to combine.
Turn the dough mixture onto a piece of plastic wrap; gather the plastic around the dough, pressing the dough into a disk. Wrap tightly in additional plastic wrap and refrigerate for at least 1 hour and up to 2 days.
Before rolling, let the dough stand at room temperature for about 15 minutes to become pliable. Lightly coat a rolling pin with flour and roll the dough on a lightly floured surface into a 15-inch circle. Lightly coat a round 9-1/2 x1-inch fluted metal tart pan with a removable bottom with cooking spray. Roll the dough around the rolling pin and carefully unroll over the tart pan. Gently press it into the bottom and up the sides of the pan without stretching. Trim the excess, leaving a 1-inch overhang. Fold the overhang inside the tart and press against the sides to make a double wall about 1/4 inch higher than the tart pan (this will offset any shrinkage during baking). Wrap the tart shell in plastic wrap and refrigerate for at least 30 minutes and up to 24 hours.
Position a rack in the lower third of the oven and heat the oven to 425°F. Line the tart shell with parchment and add enough beans or pie weights to fill the shell. Bake on a heavy-duty rimmed baking sheet until the edges are firm and starting to brown, 15 to 20 minutes. Carefully lift the parchment and weights from the tart shell. Continue to bake until the bottom of the shell is pale golden, 5 to 7 minutes more. Let cool on the baking sheet on a rack and reduce the oven temperature to 350°F.
Make the filling
In a medium bowl, whisk together the eggs, yolks, maple and vanilla extracts, and the salt. Set aside.
In a 2-quart saucepan, combine the maple syrup and the sugar and cook over medium-high heat, stirring constantly until the sugar is dissolved and the mixture just starts to boil, about 3 minutes. Reduce the heat to medium and add the butter, stirring until melted. Remove from the heat and let cool for 1 minute.
Slowly whisk the warm syrup into the egg mixture, drizzling it in about 1/4 cup at a time and whisking continuously to prevent the hot syrup from cooking the eggs.
Fill and bake the tart
Fill the tart shell with the chopped walnuts. Carefully ladle the filling over the walnuts to within 1/4 inch of the rim (do not overfill; there may be a little filling left over). Bake the tart until it is just set in the center, 30 to 40 minutes (if the crust is golden-brown before the filling is set, cover the edges loosely with foil). Cool completely on the baking sheet on a rack.
Carefully remove the tart rim. Slide a long, flat spatula between the pastry and the pan bottom and transfer the tart to a serving platter.
Make Ahead Tips
The dough disks can be refrigerated up to 2 days, wrapped tightly in 2 layers of plastic wrap.
The dough can be frozen for up to 2 weeks: Put the plastic-wrapped dough in a zip-top bag, seal, and freeze. Thaw the dough overnight in the refrigerator before proceeding with the recipe.
The finished tart can be wrapped in 2 layers of plastic wrap and stored for up to 2 days at room temperature.
The finished tart can be frozen for up to 2 weeks: Transfer it to a 10-inch cardboard cake round, wrap tightly in 2 layers of plastic wrap and then in foil, and freeze. Up to 10 hours before serving, remove the foil but not the plastic wrap from the tart and thaw at room temperature. When completely thawed, remove the plastic wrap. For freshly baked flavor, warm the tart in a 300°F oven for 10 to 15 minutes. Cool completely before slicing.
nutrition information (per serving):
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Photo: Scott Phillips