Line an 11×7 baking dish with waxed paper.
In medium bowl, mix the butter, whey protein powder, erythritol, stevia and peanut butter together. Spread mixture in the bottom of prepared pan.
In Pyrex or metal bowl over pot of simmering water, melt chocolate chips with the peanut butter, stirring until smooth. Pour melted chocolate over peanut butter mixture and spread evenly to cover top.
Refrigerate for at least an hour before cutting into squares. Keep in the refrigerator for snacking.