Blend the erythritol and whey protein together into a superfine powder. This will result in smoother chocolate.
Prepare a double boiler setup (recommended), or a small saucepan over low heat. Add cocoa butter and heat, stirring occasionally until melted.
Add baking chocolate, stirring continuously until smooth.
Add erythritol, whey protein, and stevia, stirring continuously until well mixed.
Remove from heat and continue to stir until very smooth.
At this point, you can proceed to the Keto Peanut Butter Cups recipe if desired. Or pour into chocolate molds. Refrigerate to cool until hardened. Store and enjoy at room temperature.