Bb Quartal (So What), 4th inversion (E# in the bass)
Also written as Bb quartal/E#.
Notes, from the bass up
E# (bass)A#D#G#B#
The same notes as Bb Quartal (So What) (E#, A#, D#, G# and B#), rearranged so E# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.