When I was having trouble gaining the weight, I heard great things about the Optimum 100% whey powder. I gained so much, once I started taking 2 of these a day. I just used 2 scoops of whey (52 g), 4 tbsp of PB (16 g), 2 cups of milk (16 g) and some chocolate syrup. Thats a little over a thousand calories right there, per shake. If you train hard, you wont have to worry about putting on the fat while taking these. It shouldnt be hard to reach around 4500-5000 cals a day when planned accordingly.