Bb Minor Major 9th, 4th inversion (B# in the bass)
Also written as Bbm(maj9)/B#.
Notes, from the bass up
B# (bass)A#C#E#A
The same notes as Bb Minor Major 9th (B#, A#, C#, E# and A), rearranged so B# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.