There are probably loads that reference Spike's demon wanting to claim Buffy or it feeling connected to her in some way. A couple that spring to mind...
I think it is Spike's demon that wants to claim Buffy in Wheels of Fire by greyangel, which is an outstanding fic but not available online (if you don't have it but would like to read it I can email you a copy if you pm me).
There is reference to Spike dealing with his demon on and off in
A Gem of a Soul by spuffy luvr, but that is basically representative of his darker desires/urges. Again an amazing fic though (but pretty angst driven in places).
Spike's demon seems to want to protect/connect with Buffy straight away in
Overcome by KittenofDoomage but it isn't a constant specific feature within the fic I don't think.
In the following I would say that Spike's demon plays a specific role...
In
Dreams and Nightmares by Abbyland Spike's demon is brought out but behaves protectively towards Buffy although behaving very feral, it is put in direct contrast to Angel's. (if the link to adultfanfic.net doesn't work because of the sign in process it is in BtVS AU/AR > threesomes/moresomes - if you don't normally read those it isn't a consensual situation but an induced one so it isn't the 'romance' aspect of the fic that fits that genre setting).
Taste of Juliet by Peta has Spike reduced to his feral state at one point and has a possessive/mate connection with Buffy.
Two Sides of the Same Coin by Behind Blue Eyes has the Toth split Spike and William so Buffy is dealing with the demon separately which obviously feels a connection with her.