Line a baking dish or cookie sheet with parchment or wax paper; have some excess hanging over the sides, it will help later when breaking up the bark
Add half of the chocolate (9 oz.) and half of the oil (2 tsp.) into a microwave-safe bowl and melt in 15-25 second increments in the microwave, stirring thoroughly between so the chocolate doesn't burn
Line a baking sheet or casserole dish with parchment or wax paper, and pour the melted chocolate in and spread it out evenly with a rubber spatula or knife*
Place the pan in the fridge for the chocolate to set, and prepare the peanut butter mixture while waiting
In a bowl, combine the peanut butter, powdered sugar, and powdered stevia (in a blender or food processor, blend the stevia or other sweetener up to a fine dust) and mix them together – it'll be thick, so continue to fold and press the peanut butter with a rubber spatula until there are no more clumps of sugar
Once the peanut butter mixture is prepared, disperse it as evenly as possible – it doesn't have to be perfect – over the chocolate by forming thin patties (no more than ¼" thick) by hand and layering them on top of the chocolate or pressing and spreading it with a rubber spatula*
Return the pan to the fridge and melt the remaining chocolate and oil, same as in step 2
Pour the melted chocolate on top of the peanut butter layer and use a rubber spatula or knife to spread it out evenly, covering the peanut butter entirely, then place the pan back in the fridge
Once the top layer of chocolate is set, use the excess paper hanging over the sides as "gloves" to break the bark up into pieces by hand – using the paper is far less messy as the chocolate won't start to melt in your hands
Store in the fridge (and be tantalized every time you open the fridge); it's soft enough to eat straight from the fridge, but you can let it sit at room temperature for a few minutes to let it soften even more before eating if you prefer