This one is from a tumblr post, I modified it a bit but i'll include the photos I took of the post at the end.

Ingredients:

• 1 Tablespoon Butter
• 1 Tablespoon Sugar
• 1 Tablespoon Brown Sugar
• Vanilla Extract
• 1 Egg Yolk
• 3 Tablespoons Flour
• Chocolate Chips

Any mug will work for this recipe. Put butter in mug and melt in microwave. Add in both sugars, egg yolk, and a few drops of vanilla. Mix well with fork. Add in 2 Tablespoons of flour and mix. Add choclate chips to your heart's content and mix. Add in last Tablespoon of flour and mix. Microwave for 60 seconds. The reason I add the flour kinda seperated is because its easier to add in the choclate when the dough is still kinda runny, you dont have to do that and you can just add in all the flour and then the chocolate.