Heat a saucepan to medium heat and add in ½ cup of butter. Once the butter has melted, add in the sweetener and heavy cream. Lower the heat and simmer for 15 minutes or until the mixture has reduced and thickened enough to coat the back of a spoon.
In a small microwave safe bowl, add in the chocolate chips and remaining 2 teaspoons of butter. Heat in the microwave (or over a double boiler) for 30 seconds, remove and stir. Repeat the process until the chocolate has melted.
Dollop the chocolate over the peanut butter mixture. Using a butter knife, swirl the chocolate around.
Chill in the fridge for at least 2 hours or until fully set before cutting into squares.
Be sure to use a peanut butter that has no added sugar, and preferably the only ingredient being peanuts.