Line a standard 9x13 inch cupcake tin with 12 paper liners. For easy removal, lightly spray the inside of each liner with cooking spray.
Place half of your sugar-free chocolate chips (about 4.5 ounces) into a microwave-safe bowl. Melt them by microwaving for 1 minute, then stir well. If there are still a few solid pieces, microwave for another 30 seconds and stir until mostly smooth. Alternatively, use a double boiler method for melting.
Carefully spoon the melted chocolate evenly among the 12 prepared cupcake liners, creating the bottom layer. Place the tin in the refrigerator for about 10 minutes to allow the chocolate to harden.
While the chocolate chills, combine the natural peanut butter and powdered sugar-free sweetener in a small mixing bowl. If your peanut butter is very stiff, you can warm the mixture in the microwave for about 30 seconds to make it easier to stir into a smooth, thick paste.
Once the chocolate layer is firm, remove the tin from the fridge. Spoon the peanut butter mixture evenly over the chocolate layer in each cup. Return to the refrigerator for another 10 minutes to set.
Melt the remaining half of your chocolate chips using the same method as before (microwave or double boiler).
Take the cups out of the refrigerator and spoon the freshly melted chocolate evenly over the peanut butter layer, creating the top.
Chill for a final 10 minutes, or until fully firm. Serve and enjoy! Store any leftovers in the refrigerator for up to a week, or in the freezer for up to 3 months.