Mix flour, sugar, cocoa powder, baking soda, salt, buttermilk, oil, and vanilla in a large microwave-safe mug (should be able to hold at least 16 oz of liquid).
Stir in chocolate chunks.
Dollop hot fudge sauce in the center.
Cook in microwave 60 - 90 seconds and add more time as needed. The middle will still be gooey, but the outsides should have a cake-like texture.
This recipe was tested by 3 different people with 3 different microwaves and 60-75 seconds was perfect for all of us.This recipe can be eaten by one person, but Trevor and I thought it was so rich, we actually enjoyed sharing one between the two of us with a couple scoops of ice cream!