Make the slaw: Whisk buttermilk, mayo, cider vinegar, sugar, and remaining half of scallions from the meatballs in a large bowl and add cabbage, tossing to evenly coat it. Season to taste with salt and pepper.
[Here is where, if you’re unhinged enough to make your own barbecue sauce, you can do so using the quickie recipe at the end. But there is truly no need to do this to make a great meatball slider, promise.]
Toast your buns: In a large frying pan, the same one where you’ll cook your meatballs, melt a tablespoon of butter over medium heat. Place your buns, cut-side down, in the pan. Cook until cut sides are golden-brown, about 1 to 2 minutes. Transfer to plate or platter where you’ll assemble your sliders.
Cook the smashed meatballs: Remove chicken meatball mixture from fridge and, using wet hands for a smoother shape, roll into the size and number of meatballs you’ll want — this is determined by the size of your buns. Be sure to roll the meatballs smaller than the bun so that they will fill it out once smashed.
Heat your large frying pan over high heat for a minute, then coat with about 2 tablespoons oil. Place your first few meatballs in the pan and let them sear for 30 seconds to 1 minute before using a heavy spatula to flatten them until they’re about 1/2-inch-thick. Season meatballs with an extra dusting of salt and pepper. Cook until they’re crisp and deeply browned underneath, about 3 to 4 minutes — reduce the heat to medium-high if they’re cooking much faster — and flip each, cooking on the second side until it is also browned underneath, another 2 to 3 minutes. Transfer meatballs to prepared buns and repeat with remaining meat.
To finish: Spoon barbecue sauce onto each meatball, top with a few sliced of pickle, and a small heap of slaw, and eat right away.
Quickie Barbecue Sauce: Combine 1/3 cup ketchup, 3 tablespoons each molasses and apple cider vinegar, 1 tablespoon Worcestershire sauce, and 1 teaspoon chili powder or smoked paprika (or half of each) in a skillet and bring to a simmer, stirring, over medium-high heat. Reduce heat to medium and continue to cook, stirring, for 2 to 3 minutes. Season to taste with salt and freshly ground black pepper. Pour into a bowl for serving; wipe out pan and continue to use it to toast the buns and cook the meatballs.