In medium bowl, mix the butter, whey protein powder, erythritol, stevia and peanut butter together.
Using cookie scoop, drop about a tablespoon of peanut butter mixture into silicone cupcake molds. Press mixture down into bottom of each mold. Freeze for at least an hour.
In a chocolate melter, melt the dark chocolate and keep melted on warm setting. Note: this could also be done in microwave or double boiler, but the melter is best to get the right temperature.
Remove frozen peanut butter filling from each silicone mold and set aside.
Brush melted chocolate to cover bottom and a little bit along the sides of each silicone cupcake mold. While chocolate is still melted, press the frozen peanut butter filling on top of the chocolate in each mold. Top off with a layer of melted chocolate.
Chill cups in the refrigerator to set the chocolate. Once set, the candy can be removed from the molds. The cups are fine to have out at room temperature or can be kept in the refrigerator for longer storage.
A brush or small spoon is useful for spreading out the chocolate.To ensure the sides of the peanut butter cup have chocolate, it helps to squeeze the silicone mold after topping with melted chocolate. This helps the chocolate drip between the peanut butter disk and the silicone mold.Any low carb chocolate bars, either homemade or store bought, can be used in the recipe. Keto friendly chocolate chips work too but often have added fiber to hold the chip shape.