It's called Yuutsu Na Asa. It's a ButlerXYoung Master manga (Not a Black Butler fanfic). Once he's 17 or smth the relationship grows and it's mostly about history in the 1900s. There's a scene in Volume 1 that contains R@pe and because of that I was gonna stop reading it BUT I wanted to know how their relationship would progress from there. In the beginning, the Butler seems Cold and cruel, but as you continue to read he changes. In Volume 3 and 4 You'd see the Butler acting all loving towards his master. I'm glad to say that I am completely satisfied with their relationship
I bought the 7th volume a year ago (July 2018) I'm just waiting for the 8th one to come out and buy it
edit: I got the 8th volume last year 2020 yeet
Yuuutsu Na Asa