Posting a Tumblr ficlet from a while back. Pure fluff. Enjoy!

It takes him 15 years to get the calculations right, between Gallifreyan and Earth calendars, but eventually the Doctor works out the right date, and starts planning.

Seven months later, he enters the Vault with a cake and wrapped gift in hand. Missy is draped over a lounge with the pretence of not knowing he had been coming, like the posing drama queen that she is.

"Oh, Doctor, what a nice surprise," she says airily. "To what do I owe the honour? What have you got there, is that a-"

She stops, eyes widening and voice choking on her words as she realises the implication of the singular candle stuck into the cake. Those eyes of hers go to his, with a kind of disbelief, softer than usual. Questioning if it's really the gesture it seems.

"Happy Birthday," he says softly, and grins a little at the way her mouth drops open a bit.

"But how did you-"

"Maths. Lots and lots of maths. Got there eventually."

She sniffs, trying to collect herself. "I'm surprised you even remember. Didn't think you cared so much."

"Don't be an idiot," he says, rolling his eyes. "Now, are you going to take your present or not?"

Missy snatches it out of his hands while he moves to put the cake on the table nearby. She rips the wrapping off the gift only to find herself staring at a violin case. She sits on the lounge and opens it up, pulling out the exquisite instrument inside, shiny and new. Top of the range - only the best for someone as snobby as the Master.

"Oh," she breathes, hand running over the wood in a way that is ever so slightly distracting, because she has nice hands. Not that the Doctor notices that sort of thing, because he doesn't, and definitely not more than three times a week.

"Acceptable?" he asks.

"You can't just buy me off with a musical instrument, I'm still your prisoner, here," Missy retorts, narrowing her eyes at me.

"Yes, and the alternative was you being a corpse, so get over it," the Doctor replies without missing a beat, having long since run out of guilt over the situation. "Now, get over here so we can light the candle."

"Can I?"

The Doctor knows he shouldn't trust her with any form of fire, really, but it is her birthday, so he hands her the lighter and starts mentally preparing in case arson or attack comes.

Thankfully, Missy just flicks it a few times, letting her fingers running over the small flame and biting her lip when it burns her, just a little. She likes experiencing pain as much as dealing it out, a fact not known by too many - even if its not overly difficult to guess, in the Doctor's opinion.

Finally, she lowers the lighter to the candle, and beams at it. The Doctor takes the lighter back before she can get any other ideas.

The Doctor sings a happy birthday of his own creation. It's in Gallifreyan, but a rendition of sorts of the Earth version since Gallifrey didn't celebrate birthdays in quite the same manner. There's a line about 'our people were boring about this sort of thing' in there somewhere.

Missy does that funny little delighted giggle and kisses him as he finishes the song.

"Thank you, Thete," she says softly.

"You're welcome, Koschei."

They cut the cake, and after having a slice, Missy moves to play the violin, and the Doctor gets started on his second and third pieces while the sound of Mozart fills the room.

Thanks for reading, let me know what you thought!