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Does a beverage with a cultured buttermilk like consistency break a fast?

Maybe not the letter, but probably breaks the spirit.

Let me tell you a little story...

At the seminary, there was one seminarian in the year above me who went into town one Ember Wednesday (seminarians could get permission to go into town on Wednesday afternoons, which were allocated to recreation) and bought a milkshake. He basically said "I don't care" and even joked about it.
Fast forward several years...
After this young man left the seminary, he completely apostatized from the Faith.
He also was very sarcastic about everything...like nothing was sacred or serious. One time I was sharing a sheet of funny rock group names I made up (like "Dingo Fortnight") and I commented that during my creative process, I ran my brainstorm of ideas through a mental filter to weed out any dirty or off-color ones. He responded that he preferred no such filter.

It's interesting, in a bad way, how various Trads lived BEFORE their apostasy.

Another apostate I know (same first name, oddly enough!) fell because of sins of the flesh. He openly "confessed" to Myspace things he did in his teen years, of which I had no idea. Today he's completely apostate from the Faith, and practices no religion.

Another apostate I know got into astrology and "red pill anti-Feminism" and become confused and bitter about how to reconcile it all. He suffered a lot from difficulty getting established and finding a good wife. But what destroyed his life -- both secular and spiritual -- and set him on the path away from God, was falling into adultery with a Trad married woman with many children. That totally broke his habit of weekly Mass, after the wronged husband threatened the young man's life.

All three of these apostates were Traditional Catholics. They would sing in choir and/or serve Mass, pray, and only attended the Tridentine Mass.

Now a separate issue is the necessity, recommendation for fasting every weekday of Lent (as shown on the average Trad calendar, every day of Lent is purple "fast and partial abstinence")

At the Seminary we would have no dessert at the evening (smaller) meal, the evening meal would be a bit simpler, and breakfast would be cold cereal OR bread, but not both as usual. And there were no snacks available at the 4:00 coffee break.

Nevertheless, the amount you could eat at those 2 minor meals was left between you, God, and your spiritual director. I saw some seminarians drinking a cup of coffee at breakfast and nothing more. But obviously you A) have to be able to function -- i.e., perform your duties of state, and B) not all bodies are created equal. For that matter, not all souls are in the same exact state regarding each virtue and each vice. That's why the Church leaves the particulars of the general command "Do Penance" up to each one of us.

In fact, some individuals are downright excused or advised against fasting for health reasons. Luckily there are many other ways to practice mortification. There are many things we can give up or do, plus prayer and almsgiving.

Of course the Church used to have more mandatory penance, such as fasting during Lent. On the one hand it no longer obliges under pain of sin, but on the other hand we are Traditional Catholics who root for the old disciplines and the destruction and complete rollback of Vatican II and its lax disciplines. How can we truly root for the destruction of Vatican II when part of us kinda likes it or has grown pleasantly used to it? Also, we need to do some corporal mortification to make sure our body remembers who's in charge.