Bb/B# (Bb Major over B#)
Notes, from the bass up
B# (bass)A#DE#
A slash chord names two things at once. The part before the slash is the chord your right hand plays, and the part after it is the single bass note for your left hand. Read Bb/B# as Bb Major with B# in the bass.
The bass note here is not part of the chord itself. Your left hand plays B# while your right hand plays the plain Bb Major shape above it.