In your food processor, measure the flour, powdered sugar, salt, spices and 1/2 cup of the walnuts. Pulse several times until well combined and the walnuts are finely chopped and dispersed throughout the mixture.
With the processor running, drop the butter down the chute, a tablespoon at a time, and process the mixture until moist and crumbly.
Add the vanilla extract, maple extract and milk, and process just until the dough comes together into a dough ball.
Dump the dough out onto a clean counter and gather any dry floury bits together, shaping the dough into a disk, and lightly dusting it with a little extra flour.
Line the bottom of a 9-inch round tart pan with a removable bottom with a circle of parchment paper. Press the dough into an even layer in the pan.
Use a toothpick, skewer or cake tester to prick the dough all over - this is called "docking the dough", and helps prevent it from puffing up as it bakes.
Finely chop the remaining 1/4 cup walnuts, sprinkle them on top of the dough, and lightly press them into the dough.
Now cut the dough into 12 equal wedges, using a sharp knife or bench scraper, dipping it in flour in between cuts. I used a bench scraper, which I find easiest for cutting dough inside the pan. Cutting the dough before baking makes it much easier to cut without breaking it after it comes out of the oven.
Refrigerate the shortbread for 30 minutes.