This story takes place back when the Six Kings still lived with Kuromueina in the heart of the Demon Realm.
One day, Isis and Kuromueina walked into the kitchen to find Shaltier hard at work over the stove.
"...Smells good... Shaltier... what are you... making?"
"Oh? Yeah, I just had a bit of a craving, so I whipped up some baby castella."
"Baby castella? Is that a snack from the world you came from, Shaltier?"
"Pretty much. It's a simple little thing, so it might exist in this world under a different name, but back home, we called them baby castella. Since they're fresh, do you two want to try some?"
Shaltier was a visitor from another world, and she possessed a repository of knowledge that was alien to Kuromueina and her kin. Her occasional culinary whims had become a source of great fascination for the household—including Ein, who happened to be away at the moment.
Today’s curiosity was this "baby castella," a snack neither Isis nor Kuromueina recognized. Shaltier placed a small plate of the golden-brown spheres before them.
"...Ah... it’s delicious... it has such a... gentle taste."
"..."
"Simple, but hits the spot, right? Fancy gourmet dishes are all well and good, but sometimes these basic treats are the most satisfying... Wait, Kuro-san? Is something wrong?"
While Isis was all smiles, Kuromueina had frozen in place, her eyes squeezed shut. Shaltier tilted her head in confusion, but before she could ask again, Kuromueina’s eyes snapped open.
"This is it! This is exactly it!"
"It? What is?"
"This is the snack I’ve been searching for! The bite-sized shape, that mild, comforting flavor, the simplicity and depth... This is exactly what I’ve always wanted to eat!"
"Er, okay? I’m not entirely sure I follow, but I’m glad you like it."
Though taken aback by Kuromueina’s uncharacteristic intensity, Shaltier figured she had done her job and prepared to return to her cooking. However, Kuromueina moved with a sudden, blur-like speed, firmly planting her hands on Shaltier’s shoulders.
"Shaltier! Tell me, what kind of snack is this baby castella, exactly?!"
"W-what kind? I mean, it’s just a basic baked treat made with flour, eggs, and sugar. Exactly what it looks like. If you’re that obsessed, I can teach you the recipe. It’s so easy anyone can do it as long as they have the ingredients and a mold."
"You can create this flavor so easily? Incredible... That’s simply incredible. Does it have other variations?"
"Well, the flavor profile is pretty neutral, so it’s incredibly versatile. You could fill them with chocolate or knead different flavors into the batter."
"...So the potential for growth is boundless too. Amazing... truly amazing."
"Plus, you can just toss them in a paper bag and take them on the go."
"Portable and ready to be enjoyed at a moment’s notice... W-what a revolutionary concept!"
It wasn't just Shaltier who was stunned; even Isis was staring at Kuromueina in shock. No one had ever seen her this passionate about a piece of bread.
Since the praise was becoming a bit much, Shaltier decided to see if there was a limit to Kuromueina's enthusiasm by making up some utter nonsense.
"And... if you arrange them on a pedestal and eat them under the stars, they become 'Otsukimi Baby Castella'—a refined, celestial experience."
"They even adapt to the environment?! What incredible versatility!"
"What's more, if you intentionally call them a 'Heavy Cocktail' during a conversation, someone is bound to snap back with, 'No, it's baby castella!'—making them the ultimate tool for social interaction."
"To think they even facilitate smooth communication... Their possibilities are truly infinite!"
"Wow, you really will praise anything I say. Do you honestly like them that much...?"
Shaltier watched Kuromueina with a look of utter disbelief.
It was difficult even for Kuromueina to articulate the feeling. To put it simply, she had spent an eternity waiting to find the one food that perfectly aligned with her soul, and now that she had found it, her eyes shimmered with pure, unadulterated emotion.
"The moment I tasted it, I knew. This baby castella holds the Infinite Possibilities of Baby Castella!"
"...Um... Shaltier... is that true?"
"No. 'Infinite possibilities' isn't a phrase anyone has ever used to describe a baby castella in my world, ever..."
"I've decided! It’s settled! I will dedicate myself to pursuing the Infinite Possibilities of Baby Castella! I shall seek out every variety this world can offer!"
"Yeah, okay, good luck with that. I'm gonna head out now..."
Sensing that things were about to get extremely tedious, Shaltier tried to slip away. But Kuromueina moved faster, cutting off her exit path.
In terms of raw power, Kuromueina far outstripped Shaltier. When the Underworld King was serious, there was no escape.
"...Hey, Shaltier. You're going to tell me everything there is to know about baby castella... aren't you?"
"I have nothing but a bad feeling about this! You have the exact same look in your eyes that Ein-san did when I explained the concept of maids! Do masters and servants really have to be this similar? Seriously, I wish you two would stop dragging me into your obsessions!"
"Thank you, Shaltier! Now, let us sit down and discuss the glorious future of the baby castella!"
"Were you even listening to me?! When exactly did I agree to this?!"
In a display so rare that even the veteran Isis had never seen the like, Kuromueina’s eyes sparkled as she giddily dragged the protesting Shaltier away.
Serious-senpai: "I see. So this is how the 'Devil of Baby Castella' was born... Wait, both this and the Maid Monster are entirely your fault!"
???: "How was I supposed to know she'd bite that hard?! Honestly, I still don't even understand what drives Kuro-san to go that far... If anything, I’m the victim here. A complete and total victim!"