Ask HN: Interview process - 'Exam' for empirical scoring of candidates

As part of an interview process for a senior software development role, we are being asked to create a language-agnostic 'exam' that can be sat as part of the face-to-face interview. The reason being that we will need to produce non-subjective empirical evidence as part of the justification of who is offered the role. How would you structure/format it to give the most reasonable results possible? What sort of questions would you ask?