I've spent way too much time searching for good 'Espresso Cookie x Madeleine Cookie' fics, and honestly? Archive of Our Own is the clear winner. The tagging system is a lifesaver – you can filter for exactly what you want, whether it's rivals-to-lovers, established relationship, or even coffee shop AUs. I found this one amazing slow-burn there, 'A Bitter Brew,' that nails their competitive yet weirdly respectful dynamic from the game.
That said, Wattpad has a different flavor, literally. It's where you find the super tropey, wildly creative AUs that AO3 writers sometimes avoid. I stumbled upon a fantasy knight/prince AU there that was surprisingly well-written, though you have to sift through a lot of... let's say, less polished work. The search function is terrible, so finding good stuff is half luck, half using the right community tags.
I'd avoid Quotev for this specific ship; the 'Cookie Run' fandom presence feels smaller and more focused on other pairings. Tumblr is great for headcanons, moodboards, and drabble threads, but for full-length fics, it's more of a promotional space. Your best bet is to start on AO3, use the bookmarks of authors you like to find similar stories, and maybe check the #espessed or #madeleinecookie tags on Tumblr for links to cross-posted works on other sites.