Line a baking sheet with waxed paper or parchment paper.
Chop the chocolate finely and place in a microwave safe bowl. Add half an ounce of the cocoa butter and melt in the microwave in 30 second increments, stirring in between, until smooth.
Use a spoon or offset spatula to spread the chocolate into an even layer about ¼ inch thick.
In another microwave safe bowl, melt the remaining cocoa butter and the peanut butter, stirring until smooth. Whisk in enough powdered sweetener to thicken the mixture so that it's of a thick drizzling consistency. (You should be able to still drizzle it but it shouldn't be so liquidy that it will drip all over the bark).
Working quickly, place the peanut butter mixture into a ziplock bag and snip off the tip. Pipe the peanut butter mixture onto the chocolate in spirals of varied sizes, leaving a little space between each spiral.
Starting at the center of each spiral, drag a tooth pick through the peanut butter and chocolate mixtures to create a web like effect.
Place the cookie sheet in the fridge and let harden at least 1 hour. Break into pieces with your hands.