Stuffing

Cake mix questions

So I'm starting my weight gain shakes again. I have 2 questions about cake mixes.

1.) Let's say my initial ingredients of weight gain shakes was: Heavy Cream, vanilla milk, banana(s), added butter, added cinnamon
(For taste); do I just add vanilla cake mix into the shake?

2.) how much do you actually put to make it effective?

3.) if I'm doing something wrong here what should I substitute the ingredients with.

Thank you for your time. I always wanted to incorporate cake mix. Previously my weight gain shakes helped me gain 25 lbs. I pushed towards 155lbs (heaviest weight, and I never EVER BROKE ABOVE 150 MARK, So I'm aiming towards the 160's, which is my Obesity BMI marker.
3 days