I'm interested what you disagree on!

You are right to question this part as it is a blindspot and weakly argued by me.

From my understand, and I have to admit, that I have read far more from Zizek than from Butler, so please be highly critical, Butler positions (in accordance with Hegel) that the process of becoming a subject means to be in contrast or accordance, with that what is found "outside" oneself, narratives, social norms, culture, political rule. The subject is subjugated. But out of this subjugation emerges resistance. The symbolic order is performed and in the performance subversive potential can be found. Symbols can be over and/or under played, like hyperfemininity (which in turn can also be under-masculinity in a dialectical relation) in drag for example. One aspect of the relationship between the subject and subjugation is desire. A certain desire is demanded from me by the symbolic order ("You are a woman so you have to desire men") but something may not meet up with this demand ("I am a woman but I desire woman. What am I then? What even is a woman?"). I want to argue that the "true self" enters through "the back" in their (Butlers) view on the subject. If the (resisting) subject becomes as it constitutes itself in relation to narratives/power/symbols by an excess, is this excess the true self? Is the performativity the true self? The subversion? A space opens up where the subject that is aware of their subjugation can only exist in relation to their subjugation. This "true self" is that which constantly challenges in subversive performative acts. The act of subverting has become the more authentic.

Note that my critique on Butler was derived with Zizek in mind, who at the end uses Lacans "Real" as an idea that there exists a point of non-sense. Butler challenges Zizeks use of the Real as ahistorical, while Zizek sees in relation to Lacans Real the possibility to act (oversimplified).

But I encourage to read Butler themselves on this, from what I know it can be found in "Subjects of Desire", "Gender Trouble", Contingency, Hegemony, Universality" and "Giving an Account of Oneself"