Combine buttermilk, salt, pepper, and smashed garlic cloves in a shallow dish or zip-top bag. Add the chicken breasts and coat thoroughly, ensuring they're fully submerged. Refrigerate for at least 30 minutes (or up to 8 hours for deeper flavor—I find even 2-3 hours makes a noticeable difference in tenderness). While the chicken marinates, you can prep your other components.
While the chicken marinates, bring a pot of water to a boil and gently add eggs. Boil for 10-12 minutes until hard-boiled, then transfer to an ice bath to cool completely. Once cooled and peeled, slice them and set aside. Meanwhile, cook the bacon until crispy, drain on paper towels, and crumble into bite-sized pieces. Pat the diced tomatoes dry with paper towels—this prevents excess moisture from making the salad soggy.
In a small bowl, whisk together white wine vinegar, mayonnaise, Dijon mustard, lemon juice, salt, and black pepper until combined. While whisking constantly, slowly drizzle in the extra virgin olive oil to emulsify the dressing—this creates a smooth, cohesive texture rather than a separated one. Taste and adjust seasoning if needed. Set aside at room temperature.
Preheat your grill to medium-high heat (around 400°F). Remove the marinated chicken from the refrigerator and let it sit for 5 minutes to take the chill off—this promotes even cooking. Grill each breast for 3-4 minutes per side until the internal temperature reaches 165°F. I like to resist the urge to move the chicken around; let it develop a nice char and caramelized crust for better flavor. Once cooked, transfer to a plate, cover loosely with foil, and let rest for 5-10 minutes to retain juices.
Place chopped romaine hearts in a large bowl or on a serving platter. Arrange the cooked bacon, hard-boiled egg slices from Step 2, dried tomatoes, sliced avocados, and blue cheese crumbles in rows or sections on top of the lettuce for the classic Cobb presentation. Drizzle the vinaigrette from Step 3 over the salad and gently toss to coat everything evenly.
Slice the rested grilled chicken breasts and arrange them on top of or alongside the dressed salad. Garnish with fresh chives for a bright, oniony finish. Serve immediately while the chicken is still warm and the avocado is at its best texture.