Why did Herr Starr do this to his date? What was he about to do?

Herr Starr is indeed a character from the original Vertigo series, published as part of DC's darker, grittier comics.

Sam Cotlin, the show forerunner and executive producer (along with Seth Rogan and Evan Goldberg) have conceded that they could not do a one-to-one translation from comic to episode as that would make the series end rather quickly. We cannot rely on canon for an answer here, more than likely, as there only about 75 issues in total with 4 specials and a spin-off series.

My opinion (and I will try to defend it) is that this is yet another nod to "The Big Lebowski" (Cassidy's least favorite film by the Coen Brothers). Towards the end of that film (no spoilers, hopefully), we are introduced to a group of individuals who dress in black and simply call themselves Nihilists because "They believe in nothing".

The connection to Herr Starr may be tenuous but the similarities are the following:

• German origins - The Nihilists are of German descent and we know Herr Starr initially serves a German government agency.
• Accent - just because someone is of German descent does not automatically mean they're gonna have a German accent. The choice to give both pronounced German accents is deliberate (a TVTrope, if you will).
• Philosophy - the comments to the question are what prompted this answer as there are references to sadism, which is the enjoyment of inflicting pain. In a romantic setting, it requires the consent of another party who enjoys receiving pain in a BDSM arrangement (as poorly portrayed in Fifty Shades of Gray). This, however, is not what is taking place in this scene. Sophie, per the OP, is regaling Herr Starr with a story about meaning, which is the antithesis to Nihilism. In asking her to remove her top and stick the butter under her chin, he is showing that a) he has no interest in her story b) demonstrating that his actions won't always have a purpose (as he might not believe in anything either) c) as someone who doesn't believe in things like morality or propriety, actions that might be deemed cruel to a casual observer are part and parcel to any conversation with him.

There are no pointless scenes in television, every minute (indeed every second) is paid for in actor salaries, wardrobe, makeup and post-production. Whether the choices that are made make sense to each of us individually or to the audience as a whole is another topic altogether.

Update after re-watching scene

This scene occurs in the Herr Star chronology after he's shown the video of Jesse using the voice of God super-power on Tulip, telling her to "Sleep" Everything else said above still stands, but with the additional context of Herr Starr being unimpressed with the footage, saying he's bored (after the floating pig incident) "Since when is a woman obeying a man a superpower?". The dinner scene then is him proving his own point. Power is not always supernatural, the daughter of the governor of Louisiana felt like she had to listen to what he said as he worked for the world's most powerful organization, with agents in 113 countries, all ready to do his bidding at a moment's notice. Would you refuse what he said?

Next on the chronology, we do hear about his R-word fantasy with multiple prostitutes but through his battle with Cat popup ads, he accidentally reviews more of Jesse's surveillance footage.

Once again, everything above stands but perhaps Herr Starr is a bored Nihilist searching for the same thing Jesse is - order in a world of chaos.

One more update - within the larger context

I am getting tired of the phrase Nihilism because it's a weird word to type, like bananas, too many of the same vowel- I digress.

In their initial date, mentioned by the OP, Herr Starr was more than bored. He was baffled perhaps even offended by her story, perhaps the way she told it. The conversation starts with how important Herr Starr is and how his influence got them a reservation at a restaurant that was closed other than the two of them (and the staff, of course). Sophie, impressed with his masculine power/influence, tries to regale him with her feminine accomplishments of traveling and volunteering but it is the way she tells it that offends him. She begins with a condescending tone of someone who has achieve enlightenment and has to teach the ignorant "The Way" or some such ("I was once like you were..."). Indeed, what triggers the sequence of events that started this question and its lengthy answer (I'm sorry... honest) is the idea that a boy with a traumatic past finally smiled because she gave him a piece of chocolate and showed him KINDNESS, which was her missing puzzle piece.

Rather than boredom, he challenges her point of view with disdain. ("A half-wit's smile was your missing piece? You do know they smile all the time?"). Then the sequence of events from the OP take place.

Fast forward in his chronology, during the misunderstanding of what Herr Starr actually wanted and what he got from Featherstone and Hoover from NOLA brothels, Herr Starr actually is forced to focus on his desk and Jesse Custer's name and repeats the phrase Sophie said to him during their date "The Missing Puzzle Piece". Not to be too graphic or philosophical but in wanting to assert his masculine energy, the three professionals instead forced him to take a more ahem submissive role and in accepting this ahem position was able to see his boredom from a different perspective. Perhaps Sophie was on to something?

The Episode concludes with Herr Starr meeting Jesse at a bar. Jesse, as you may recall, missed a critical clue from the audition DVD, which was that it was property of The Grail, the organization that Herr Starr works for (Ironically, he'd been praying for a sign while their commercials played right behind him at the Docu Dorks waiting area). We see the contrast of the white suit that Herr Starr always sports to Jesse's black Preacher ensemble.

Thank you for reading - as a reward, I leave you a mild spoiler that I don't think breaks the story in any way but credit to IMDB

"Jesse Custer" is an anagram for "Secret Jesus" - which is the character we next get to meet in the next episode, Jesus and the secret that he asks the "ailing" woman to keep.