Is the following scale correct for C## Harmonic Minor?
C##, D##, E#, F##, G##, A#, B#
I read on a good music theory page and they had B## as 7th note in the key, so I was wondering, but I'm pretty sure mine is correct with B#.
Close; you actually do need Bx (B doublesharp) instead of just B♯. B♯ would be the minor seventh above Cx, and since harmonic minor has the major seventh (also called the leading tone), it must be Bx. Your scale is actually the Cx natural minor scale.
It's hard for me to imagine an instance where you might need the Cx (x being a doublesharp) harmonic minor scale, but there's one quick trick when dealing with difficult accidentals:
Spell it without the difficult accidentals, and then add the accidental(s) back to each pitch.
So instead of spelling the Cx harmonic minor scale, let's instead spell the C♯ harmonic minor scale, which is much easier:
C♯ D♯ E F♯ G♯ A B♯ C♯
Since that scale starts with C♯ and we want Cx, let's just add a sharp to each pitch, because that's what we do to get from C♯ to Cx:
Cx Dx E♯ Fx Gx A♯ Bx Cx
Or, maybe you'd rather start with the C harmonic minor scale:
C D E♭ F G A♭ B C
In which case we would add two sharps to each pitch. E♭ and A♭ would then pass through E♮/A♮ before proceeding to E♯/A♯.
This trick works for anything: scales, intervals, chords, you name it. So if you ever find yourself having trouble spelling a D♯ major triad, just think of a D major triad instead:
D F♯ A
And just add a sharp to each pitch:
D♯ Fx A♯