I remember reading about the characteristics of the or a Buddha, and one of them was “a Buddha is always focused on voidness.” Sadly I did not save the website and don’t remember where this was written. Now to the question. What are the mental characteristics of a Buddha? Can you give me a list? I don’t mean the nine qualities. It is a different list, a less common one. It should include the point I mentioned and be of mahayana origin.
You've asked for a Mahayana text, which may or may not include MN121, in which the Buddha says:
MN121 Whatever ascetics and brahmins enter and remain in the pure, ultimate, supreme emptiness—whether in the past, future, or present—all of them enter and remain in this same pure, ultimate, supreme emptiness.
Although "emptiness" is probably what you meant by "void", it's worth noting that Early Buddhist Texts (EBTs) discuss several terms which can get conflated with "void." Let's look at some of these:
AN9.41:11.2: ‘Why don’t I, going totally beyond perceptions of form, with the ending of perceptions of impingement, not focusing on perceptions of diversity, aware that “space is infinite”, enter and remain in the dimension of infinite space?’
AN9.41:13.2: ‘Why don’t I, going totally beyond the dimension of infinite consciousness, aware that “there is nothing at all”, enter and remain in the dimension of nothingness?’
SN41.6:3.2: “But sir, how does someone attain the cessation of perception and feeling?”
The above three experiences are differentiated and transitional states leading to the experience and practice described in MN121:
MN121:3.3: ‘Ānanda, these days I usually practice the meditation on emptiness.’