Put walnuts and 1/4 cup of sugar in a small food processor and ground until the nuts are finely ground. Set aside.
In a bowl of an electric mixer with the paddle attachment, cream butter, and the remaining 1/2 cup sugar together for 3-4 minutes. Add the egg yolk, maple syrup, and vanilla extract and beat until well blended.
In a separate bowl, whisk together flours, walnuts, and salt. At low speed, gradually add the flour mixture into the butter mixture and stir until just blended.
Divide the dough in half and shape into 2 disks. Wrap each in plastic wrap and refrigerate for at least 2 hours until firm.
Preheat the oven to 350?F.
On a lightly floured surface, roll out the dough into a thickness of 1/8-inch. Using 2-1/4-inch leaf cookie cutter, cut out as many cookies as you can.If the dough seems too soft, chill them for 10 minutes before you bake. Gather up the scraps, reroll them, and cut out more cookies. Space the cookies 1-inch apart on a cookie sheet.
Brush the top very lightly with egg wash and sprinkle with a generous amount of turbinado sugar. Bake the cookies in the oven for 10-12 minutes until the edges are turning slightly golden. Keep the rest of cutout dough in the fridge while the first batch is baking.
Transfer cookies onto a wire rack and cool completely.