They're cooked. They're just sugary and gross.

I read this as they are inspired by cosmic brownies, not made from them. It wouldn’t be unheard of, but blending an already baked and processed brownie into a drink seems…bad. You asked if they were spoiled, but these type of processed baked goods don’t really spoil like that generally because of all the preservatives.

https://www.littledebbie.com/cosmic-brownies

They're not good. But they're supposed to be that way. I've never had anything quite like them outside of the US so it's hard to think of something to compare them to that a European might be familiar with. They're brownies in the same way that French fries are vegetables. Technically... Yeah... But no. They're extremely sweet, taste quite strongly of artificial chocolate which may be a flavor unique to American paletes, and technically resemble a fudgey brownie.

All that to say, this shake did not contain this particular garbage snack.

I suspect what he made was a protein shake with added fiber and artificial sweetener. You can get cosmic brownie flavored protein powder. Protein shakes certainly can have an effect on the gut.

Neither. It was something else. (Likely fake).

Brownie? The cookie thing?