Position racks in the upper and lower thirds of the oven and preheat to 350 degrees F. Spread the walnuts on a baking sheet and toast in the oven until golden, 8 to 10 minutes. Let cool, then transfer to a food processor and pulse until finely ground.
Whisk the ground walnuts, 1/2 cup confectioners' sugar, the flour, baking powder and salt in a medium bowl. Beat the butter in a large bowl with a mixer on medium-high speed until smooth, 2 to 3 minutes. Beat in the egg yolk, maple syrup and maple extract until incorporated. Reduce the mixer speed to medium low; add the flour mixture and beat until the dough comes together, about 2 minutes.
Line 2 baking sheets with parchment paper. Form teaspoonfuls of dough into balls and arrange about 1 inch apart on the prepared baking sheets. Bake, switching the pans halfway through, until golden, 18 to 20 minutes.
Remove from the oven and brush the cookies with maple syrup; let cool 2 minutes on the baking sheets. Put the remaining 2 cups confectioners' sugar in a shallow bowl; add the cookies in batches and toss to coat, then transfer to racks to cool completely.