B - root
D# - major-third
(F# - perfect-fifth, possibly omitted)
C - flat seventh
F - sharp eleventh extension
I've updated my chord inversions PDF to include fingerings for dom.7#11 .
What to play over a seven #11?
Lydian b7. The most 'appropriate' scale to play is something called the Lydian b7. The Lydian b7 scale is equivalent to the Mixolydian with a #4 (instead of Mixo's perfect 4). This gives you the tones of the root, 2, 3, #11, 5, 6, b7.
Altered Scale. As with any altered dominant, the altered scale is a good choice. The altered scale gives you the root, b9, #9, 3, #11, #5, and b7. For B7#11 you can play B Altered.
Symmetrical Diminished. The symmetrical diminished on get you the root, b9, #9, 3, #11, 5, 6, b7. For B7#11, you can play B Sym-Dim.
Whole Tone Scale. Another interesting idea is play the whole tone scale. This gives you the root, 9, 3, #11, #5, and b7. For B7#11, play B Whole-Tone.
Melodic Minor at the 5th. Playing the Melodic Minor starting on the 5th from the root will give you root, 9, 3, #11, 5, 6, b7. For B7#11, you can play F# Melodic Minor.
Altered Scale at Tritone. Playing the Altered scale based on the tritone of the root is the same as the Melodic Minor of the 5th -- it gives you the same chord tones. For B7#11, you can play F Altered.