I'm hoping you guys can point me in the direction of a fic similar to Black Silk by Nicky or even one similar to My Brother's Best Friend by Dampersandspoons.

To sum it up, I'm wanting NC-17 fics where (a young or innocent) Buffy ends up with Spike with this playful edge: e.g. through a game of spin-the-bottle, truth-or-dare, alcohol/substance-induced silliness/dares.

...Or where Spike is seemingly out-of-reach; a brother's friend, or even her step-brother. I.e. they think it could never happen, but it does!

It can be long or short fics, and they can get together right away, or it can take a while. I also don't mind if it's a one-shot fun fic or if they end up in a long angsty but ultimately lovey-dovey relationship.

Also, I don't mind the genre. I've read a few AU fics like this, but ones where Spike is a Vampire and Buffy is the Slayer would be well-revieved too

Smile by Vanilla might fit the bill. Buffy has this adorable, secret crush on Spike, it's similar to the My Brother's Best Friend. It's very well written and I highly recommend it! She gets drunk at a party, which then results in her and Spike getting together after she sobers up.
Summary: Buffy knew it was just silly. Most seventeen-year-old girls had moved far beyond the stages of obsessive, unrequited crushes, had long since stopped signing fake names in their notebooks. But she just couldn't help it.

Then I can recommend a story with Slayer Buffy and Vamp Spike though I'm not so sure Buffy is as innocent as in the human fics you mentioned. It is however, focused on dares.

The High Stakes Series by Chelle is a fun, hot read.
Summary: Buffy and Spike try to come to terms with working with one another by making bets. It starts out harmless enough, but quickly becomes something neither of them are prepared for.

Are there any others? I liked these stories, especially Vanilla's, which was adorable -- but it was too short! So, any longer ones on this theme out there? I'm particularly interested in the idea of a Vampire Spike coming across an innocent or relatively innocent Buffy and not hurting her -- but anything along these lines will suit, including all human fics about relationships.

Sideways by Golden Usagi. It's long and Buffy is relatively innocent.

The Enemy of Reality
I just finished reading Smile by Vanilla - thank you so much for suggesting it, it's an amazing fic! :)
I've read the High Stakes Series a few times now - also a great read!

Any others?! :D

Happy you liked it, it's one of my all time favourites. I remembered other fics you might like... Obsession by Holly has innocent Buffy with a crush on Spike. It's more of a PWP/one-shot, but it's definitely worth a read.
Summary: After Buffy catches him in the middle of an intimate act, Spike decides punishment is in order.

In Omne Tempus by Holly is also amazing. It's a longer AU fic, where Buffy is innocent since Angel isn't there to take it from her. You might like it.
Summary: For a hundred years, William the Bloody has led a trail of bloodshed and chaos across Europe and the Americas. That all comes to an end when the woman he's devoted his existence to brings his mate to him in the guise of a late-night snack. A small girl with eyes of green and blonde hair. And suddenly, Spike is thrown into a world of color beyond the black and white, and his life is never the same.

Since you liked Smile, you'll probably like Kisses and Cupcakes from Vanilla as well. S2 fic, Spike comes across unhappy Buffy, there's alcohol and truce involved. It's a great read!
Summary: Days after the tragedy that was her seventeenth birthday, Buffy sits alone on her porch. An unexpected visitor becomes her partner in misery, and together they come to understand how thin the line is between love and hate.

Hope you'll like!

Tempesti de Amore by Holly.

Who Are You Calling Slave by Silly_Bint on this site.

Starts with Extremely drunk Truth or Dare, ends with Spuffy.