Question

Is there a configuration hook to make the “Buffers List” buffer automatically closing when a buffer is selected in such window?
Each time the buffer list opens and I select a buffer I have to manually close it using
C-x C-k
and this is annoying, also because it leaves the frame split in at least two windows.

Answer 1

Not sure about a config option, but you can always use q (or a C-x 1 in your current buffer) instead of C-x C-k to close the temp buffer.

P.S. Almost nobody uses buffer-list these days. Most Emacs users simply remap it to ibuffer (its much smarter and you might like the fact that its temp buffer disappears by default):

(global-set-key (kbd "C-x C-b") 'ibuffer)