In medium mixing bowl, cream together butter and granular sweetener (I used Sukrin:1) with electric mixer.
Stir in egg and vanilla.
In small bowl, combine almond flour, coconut flour, whey powder, baking powder, and salt.
Stir dry mix into wet mix.
Mix in white chocolate and macadamia nuts.
Scoop tablespoonfuls of batter on silicone or parchment paper lined baking pans. Bake at 375°F (Comments noted this might be too high. May want to try 325-350°F instead) for 8 to 10 minutes (edges should be nicely browned).
Cool for five minutes, then transfer each cookie to wire rack to cool completely.
Store covered in refrigerator. Best served next day (they will be crumbly right after baking).
Notes
Each cookie contains about 4.3g erythritolCookie dough can be frozen if desired to bake later.