You can just put the shake into the brownie mix and make them as usual, this to me has the best overall quality as you know all THC is in the brownies but having the plant fiber in your brownies also makes them very chewy.
You could just heat up some cooking oil or some butter(mmmmmm) to a very low temp as to make it warm to hot, but not to hot that it vaporizes the THC and just mix your shake with it until the butter gets very green, it shouldnt take long. THC is essentially an oil so it readily dissolves into oils with a little help from heat. Then just use the oil or butter in your recipe.
As far as amount? I usually like to put 3-4 grams in a 9x9 pan and make em real thick. Also if your going to just blend your shake with your brownie mix I would get a super yum white choco fudge nut peanut butter sprinkle brownie mix and not just "great value" from wally world. As to hide the fibrous consistency of the herb and also because it just goes so well with cannabis flavor and aroma.