Preheat oven to 200 degrees. Line 3 baking sheets with parchment. Using a cake pan or a ruler, trace a 9-inch square on each piece of parchment.
Whisk walnuts, 3/4 cup superfine sugar, the confectioners' sugar, and salt.
With an electric mixer fitted with the whisk attachment, beat egg whites on medium-high speed until soft peaks form. Gradually add remaining 1/4 cup superfine sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold in walnut mixture with a spatula. Spread batter evenly onto traced squares on parchment, and smooth tops.
Bake for 2 1/2 hours. Turn off oven, and let cool in oven until completely dry, about 75 minutes.
Carefully peel meringue layers from parchment, and let cool completely on wire racks. Meringue layers can be stored in airtight containers at room temperature overnight if the humidity is low.