What is one of the fastest ways to gain weight but in muscle and not fat?

Gaining muscle: HIIT: High Intensity Interval Training. Google it for more. In short lifting heavy weights in a very short period of time (15 minutes rather than an hr) will help release more Human Growth Hormone. Supplement weight lifting with intake of 100% grass fed protein. Grass fed beef is best source but powdered whey protein is an option. Check out Paleo Diet. Abstain from sugars "The enemy tastes sweet."