Wow!!, this looks like very easy... May go and try as well. And sound tasty... hehe
Yea it can be really tasty depends on what you mix. You can go as arguably "bad" as Whey + ice cream or Whey + milo and it is still cheaper and tastier than any commercial Mass Gainer
The only good about Mass Gainer I can see if you really want me to convince you to buy one is they can be keep for long period and convenient.
Some simple concept Build muscle = resistant training + progressive overload + eat more than you need Build muscle without building fat = resistant training + progressive overload + eat more you need + monitor and optimize your calorie surplus and keep trying (everyone one is different) Loss fat = resistant training + eat less than you need Loss fat without losing muscle = resistant training + eat less than you need + monitor and optimize your calorie deficit and keep trying (everyone one is different)
+weight = eat like a monster -weight = eat like a begger
The most challenging part is optimization and sustainable to your lifestyle, be it training or eating. Some people eat little and gain real fast Some people eat shit load of foods and gain shit Vice versa
Calories calculator is a reference point to start
Some people has no life stress, they can train like athletes. Some people has shit loads of stress from life, gf, family, work, money, they cant cope more stress coming from training or they will fatigue themselves pretty fast. Some people underestimated their capability and undertrain, vise versa