Place butter and peanut butter in a medium-size bowl. Microwave, stirring every 30 seconds until completely melted.
Add sweetener, stevia and vanilla. Stir until smooth. Pour into a 8x8 pan lined with parchment paper. Place in freezer for 10 minutes.
Break chocolate into pieces and place in a small bowl. Melt in microwave, stirring every 30 seconds until just melted, careful not to scorch. (may also use a double boiler for this step)
Remove peanut butter layer from freezer. Pour chocolate over peanut butter and use the back of spoon or spatula to evenly spread the chocolate. Return to freezer for 1 hour. Allow to sit out for a few minutes before cutting.