Why Can't We Resist Chocolate? A Food Scientist Explains 

Yay, it's already in the works!

Reading this comment absolutely made my morning :) Thank you!

That's a great question! You can't use butter because it's about 20% water and water makes chocolate "seize." It sort of gets all solid-like with even a small amount of water added. BUT, in milk chocolate especially, milk fat is used and has a great mouthfeel!