In a food processor, whirl walnuts until finely chopped. Add flour, maple sugar, baking powder, and salt; whirl to mix. Add butter and almond extract; whirl just until dough holds together. (Or whirl nuts in a blender until finely chopped, or finely chop nuts with a knife, and place in a large bowl. Add flour, maple sugar, baking powder, and salt, and stir to combine. Add butter and almond extract, and rub with your fingers until fine crumbs form. Pat dough into a ball.)
Shape dough into 1 1/2-inch balls and set 2 inches apart on 14- by 17-inch baking sheets. With the bottom of a glass or your palm, flatten balls to about 1/2 inch thick.
Bake in a 325° oven until cookies are golden, 23 to 25 minutes (about 30 minutes in a 300° convection oven); if baking more than 1 sheet at a time, switch pan positions halfway through baking. With a spatula, transfer cookies to a rack to cool. Serve warm or at room temperature. Store cool cookies airtight up to 3 days; freeze to store longer.
Nutritional analysis per cookie.