2 tablespoons (20g) maple sugar, for topping; optional
Preheat the oven to 350°F. Lightly grease a large baking sheet or line with parchment.
Place the walnuts in a single layer on the baking sheet and bake for 8 to 10 minutes, until light golden brown and fragrant. Remove from the oven and let cool on a rack.
In a large mixing bowl, beat together the eggs, granulated and brown sugars, maple syrup, and maple flavor. Add the melted butter and mix until smooth.
Stir in the flour, baking powder, and salt until combined, then add the nuts.
Divide the dough in half and, using wet hands, shape each half into a rectangle about 12˝ long, 3˝ wide, and 1/2˝ thick. Sprinkle the top of each rectangle with a tablespoon of the maple sugar.
Bake for 30 minutes, until light golden brown and firmly set. Remove from the oven and let cool on the pan for 10 to 15 minutes.
Carefully transfer the baked logs to a cutting board and, using a serrated knife, cut the baked dough on a slight diagonal into 1/2˝-thick slices. Saw the knife back and forth instead of pressing down, to keep the cookies from crumbling.
Place the slices close together, standing upright on the baking sheet. Return to the oven and bake for another 20 to 25 minutes, until the sides begin to brown. Remove from the oven and let cool on the baking sheet on a rack.
Store airtight at room temperature; they'll stay good for weeks.