Separate egg whites from the yolks. Measure out approximately ½ of an egg white (1 Fl. oz.) and keep it separately.
Preheat the oven to 350 °F (175 °C). Line a baking pan with parchment paper. In a medium bowl, using an electric mixer, mix softened butter and erythritol. Add in vanilla extract and mix.
Using an S-blade food processor, ground the macadamia nuts. Make sure you leave it roughly ground, not turning it into flour or even butter.
Put the ground macadamia into a medium bowl. Sift together both flours and salt, adding them to the ground macadamia.
Add the flour mixture and macadamia to the butter-erythritol mixture. Mix on low speed until the dough starts to come together.
The dough now is probably crumbly and does not hold together very well, but do not worry. Shape it into a flat disk, wrap it in a plastic foil, and let it rest in the fridge for 30 minutes.
Roll the dough on the baking pan ½ inch thick. It still acts crumbly at this point but will hold together once baked and cooled down. Try shaping a square. Cut out 12 or more rectangle-shaped cookies.
Take half of the egg white, mix it lightly with ½ tbsp. powdered erythritol, and spread it over the cookies.
Bake for 20 - 25 minutes. Make sure not to burn the cookies. Take them out of the oven and let them cool down completely. They will get firm when cooled. Once cooled, crush the cookies.