Melt peanut butter and butter in a bowl in the microwave for 10 seconds. Add erythritol and mix carefully.
Melt chocolate and coconut oil in a double boiler. Pour half of the mixture into a mini muffin baking pan.
Roll the peanut butter mixture into balls and flatten a bit. Put on top of the chocolate in the pan. Pour remaining chocolate on top.
Place in the fridge for at least 2 hours.