From cjnwriter: Baking with Mrs. Hudson
It wasn't a secret, Mr Holmes told her defensively when she asked. She quirked a doubtful eyebrow, but he didn't elaborate, and so the matter was dropped.
"Sift the flour," she said, partly to change the subject and partly because his last attempt had been lumpy. He flashed a look of gratitude and followed her instruction.
"My mother taught me this recipe," she told him as he took the spoon and started mixing. "It will rise more with the baking powder, but we never had any of that back then. This is how I'm used to making it." [1]
"What measurements for the milk?"
"Mr Holmes," she chastised. "You know what I think about that. You just-"
"'Eyeball' it," he quoted with a look of distaste at the cake mixture. "That did not end well for me last time."
"It's just practice," Mrs Hudson dismissed. "You have to get it wrong before you can get it right."
The look he drilled her with made her laugh out loud. She could picture the sort of boy he was in moments like this. Precocious, perfectionist... And, she had a very strong feeling, unloved.
"That's enough," she instructed sharply, batting his hand so he stopped pouring. "Now mix that and I'll grease the trays."
"Have you ever tried it with the baking powder?" He looked up from his bowl to ask. "Perhaps we could experiment."
"I am not a great fan of your 'experiments', Mr Holmes," Mrs Hudson reminded him. "Let's see how this goes, and decide afterwards."
"You have exceeded yourself, Mrs Hudson!" Doctor Watson exclaimed as he ate a slice of the Victoria sponge. "Holmes, you must try some of this. I believe this is her best yet!"
"I may try it later, Watson."
"Your loss," Watson rejoined happily, taking another slice.
Mrs Hudson thanked Watson for his praises, but watched Holmes carefully. His eyes flicked swiftly up, to watch Watson's reaction to his cake, but as they did he locked gazes with Mrs Hudson who smiled knowingly. His cheeks flamed red and he ducked his head back behind the newspaper.
"Yes, I used baking powder this time," Mrs Hudson said to Doctor Watson. "A successful experiment, I'd say."
She still didn't know why Mr Holmes wanted to keep their baking a secret. Perhaps embarrassment, for it did not exactly fit with the cold image he put forward of himself. Or perhaps, she reflected later in her bedroom as she flicked through her mother's old recipe book for something new the detective could 'experiment' with, he just enjoyed their time in the kitchen together as much as she did and would prefer no one else intrude.
[1] Baking powder was only introduced in the mid-19th century, and is the reason the famous Victoria sponge has its rise! Beforehand, it was a much flatter cake.