Break the chocolate into pieces and put in a microwave-safe bowl. Add the coconut oil. Microwave on High in 30-second increments, stirring between each time, until the chocolate and coconut oil are melted and combined.
Divide the melted chocolate into 2 equal parts (put half in another bowl). Half of the chocolate will be for the bottom layer of peanut butter cups and the other half for the top layer.
Using a spoon, distribute half of the chocolate between the paper baking cups of the muffin tin. Put the muffin tin in a freezer for 10 minutes.
Meanwhile, put the peanut butter and Erythritol in a microwave-safe bowl. Microwave on High for 30 seconds, then stir until combined.
Remove the muffin tin from the freezer and spoon the peanut butter mixture into the paper baking cups on top of the chocolate layer. Return the muffin tin to the freezer for 10 minutes.
Remove the muffin tin from the freezer. Using a spoon, add the remaining chocolate to the peanut butter cups to make a top chocolate layer.
Freezer the keto peanut butter cups for 30 minutes, or until fully firm. Store in a refrigerator or freezer.