Spike becomes William but in vamp body

I just remember this part where he forgets and goes into the sun and lights himself on fire. Buffy freaks out and yells at him so he gets mad, but since a Victorian gentleman would never yell at a lady he's polite, very polite and it annoys the hell out of her. Dawns in the story and I think willow already tried to end the world

Could it be this one?

Strip Snap----- by Lilachigh

Shadows babe
no that's not it

Is it Confluents by Enigmaticblue?

Guest
That's it, thanks!