Kurogane had made a small mess in his kitchen as he worked on making a treat for a pair of twins. Normally, he didn't bake anything sweet, since he didn't like sweet foods. But that day was an exception. Half the ingredients seemed to have something on his should not to do not eat without getting sick list, but the twins loved them, so just this one day he'd make them a whole batch (and already knowing there was a good probability at least one of them would find a way to get him to eat a bite) of fudgy brownies.
He checked on the brownies occasionally as the fudgy goo solidified in the oven, making sure not to burn them. He'd have to start all over then, because he was not about to give the Fluorite twins burned brownies on their own birthday.
Of course, he pretended he didn't know when their birthday was, not that Fai had shared that information and neither had Yuui, but he'd been paying attention and taking cues from their behavior. So when he had called on the day he suspected was their birthday, asking if they could hang out that day (odd enough of him to be doing the calling) and Fai had acted very suspiciously (by trying not to seem suspicious only made him seem more suspicious).
Once the brownies had finished, and cooled down some, getting covered up, he called Yuui, asked if he could drop something off at his place that he'd borrowed a while ago (the cake pan that now held the brownies - he borrowed it to bring leftovers to his place and hadn't quite gotten around to retuning it, intentionally so he could use it some days later). Yuui told him if he wasn't home at the time, he could just let himself in and all, since he had a key to his apartment after all (they each had each other's spare key) and he'd promised to take his Brother out to diner that night. So once he's sure the two have made their way to dinner, does Kurogane knock on Yuui's door. Getting no response, he uses the key he'd been given and murmuring a quiet exuse me, lets himself in the apartment. Since they were next door neighbors, he didn't even wear shoes over, so walked inside and left the pan of brownies on the kitchen counter where he was sure they would be quite obvious to the blondes. The note left with them would help: To the Fluorite twins. Happy birthday. He left, locking the door behind himself.
Hours later, he gets a call while he's out for a jog. He paused in his jog to answer the call, noticing it was Yuui's number.
"Hey Kuro-sama-sensei, where are you~?" Kurogane could tell it was Fai, and more than a little drunk at that, "I knocked on your door but nobody was home."
"That's because I'm outside, getting some exercise."
"When are you coming back? Nii-chan and I wanna see you."
He could hear on the other end Yuui trying to take back his phone with an embarrassed sounding "Brother…I'm sure Kurogane-san is busy…"
"K-Kurogane-san…?" And Yuui had reclaimed his phone back from his half way to drunk brother, who began mewing and hiccing in the background.
"How much has your brother had?"
"A few bottles," Yuui sheepishly admitted.
"A few?"
"Five… although he shared some with me so maybe closer to four…"
There was a short sigh, "He's been getting into my sake, hasn't he?"
"Yes, how did you know, and how did you know it was our birthday? We never said when-" Yuui stopped himself there, because he just gave Kurogane all the confirmation he needed to ascertain it was their birthday that day.
"You two have been acting weirdly… so I knew something was up. Glad to know I was right. I'll be home in an hour or so. Try not to let your brother destroy the place before I get back."
"Of course, Kurogane-san."
"Oh and, ah, happy birthday Yuui."
Kurogane hung up and went back to his jog while Yuui's cheeks were as red as dinner's wine. Kurogane almost never called people by their name, much less first name.
When Kurogane returned, he put the two blonds to bed, one easier to tuck in than the other, and then hid the ekibabe. May they have a happy birthday, and Fai have a terrible hangover the next day until he took pity on him and let him have some medicine.