Melt cocoa butter over very low heat, or in the top of a double boiler.
Remove from heat. Ensure that the cocoa butter is no warmer than room temperature.
Add powdered sweetener, 2 Tablespoons at a time, stirring well to combine after each addition.
Add protein powder, if using, stirring well to combine after each addition.
Open vanilla bean, if using. Scrape the insides of the bean with a knife. Add scrapings and salt to melted cocoa butter mixture, stirring well to combine.
If necessary, pour mixture into a small bowl and process with an immersion blender. Alternatively, place in a blender and process at low to medium speed to combine.
Pour blended mixture into an 8x8 baking pan (really, any size will do) and let sit to cool. Alternatively, place in refrigerator or freezer to accelerate cooling.
Using a knife, gently break the hardened mass into large pieces and remove from baking pan.
Place large pieces of chocolate bark onto a cutting board and cut into desired-size pieces.
Store in an airtight container (if you don't eat them all!)
For AIP, some say that cocoa butter should be acceptable for all except those with a severe sensitivity to chocolate, but an option that I have not tested is to replace the cocoa butter with 1/2 coconut oil and 1/2 coconut butter.