Heat the oven to 400°F. Line two rimmed baking sheets with foil for easy cleanup.
Arrange the bacon in a single layer on one sheet. Bake until browned and crisp, 12–18 minutes, then transfer to paper towels to drain and crumble once cool enough to handle.
Meanwhile, make croutons: On the second sheet, toss the bread cubes with the olive oil and the kosher salt, spread in an even layer, and bake until golden and crisp, 8–12 minutes, tossing once halfway. Let cool.
Make the dressing: In a small bowl, whisk the mayonnaise, buttermilk, lemon juice, Dijon mustard, garlic, and the black pepper until smooth and pourable.
In a large salad bowl, combine the romaine lettuce and cherry tomatoes.
Add the croutons and most of the crumbled bacon, reserving a little for topping.
Drizzle about two-thirds of the dressing over the salad and toss gently to coat.
Taste and adjust the dressing or acidity if needed.
Transfer to plates, spoon on more dressing to taste, and top with the remaining bacon.
Garnish with chives and serve immediately while the croutons are still crisp.