I ask this question - always - but it goes like this.

Q1: Rate yourself 1-10 on X language? Their answer is Y (and almost always 7).

Q2: What's the difference between a Y and a Y+1?

Q2 is real question with Q1 being there just for framing. Some people simply state that the difference between a 7 and an 8 is "just more experience". TBC, that's not a wrong answer.

However what gets me really excited is when this line of questioning shines a light on what the candidate is curious about, or interesting in learning next. It goes a step beyond just self-awareness that there is always more to learn. To me it's a signal that this person has a plan and a drive to grow oneself.

While I do not make binary hiring decisions on this question or any question; I can vividly remember the answers I was given to this line of questions by some of the best engineers I've worked with.