Make ranch dressing: Whisk together mayonnaise, sour cream, buttermilk, dill, garlic powder, onion powder, salt, pepper, and lemon juice in a medium bowl until smooth. Set aside at room temperature while you cook the chicken and bacon.
Cook bacon in a large skillet over medium heat for 8 to 10 minutes, flipping occasionally, until crispy. Transfer to paper towels to drain. Pour off most of the bacon grease, leaving about 1 tablespoon in the skillet.
Pound chicken breasts to ½ inch thickness between plastic wrap. Season both sides with garlic powder, onion powder, salt, and black pepper. Add 1 tablespoon butter to the skillet with reserved bacon grease. Cook chicken over medium heat for 5 to 6 minutes per side until golden brown and internal temperature reaches 165°F. Transfer to a cutting board and let rest 3 minutes, then slice each breast in half to create 4 portions.
Slice croissants in half horizontally. Spread remaining butter on the cut surfaces. Heat griddle or clean skillet to medium-low. Place croissant halves cut-side down and toast 1 to 2 minutes until golden brown. This prevents sogginess from the ranch.
Assemble croissants: Spread 1 to 2 tablespoons ranch dressing on the bottom half of each toasted croissant. Place one chicken breast portion on each bottom. Top chicken with 2 strips of crispy bacon per croissant. Sprinkle ¼ cup shredded cheddar cheese over the bacon.
Optional: Place assembled bottoms (without top bun) under broiler for 1 to 2 minutes to melt cheese, or return to griddle and cover with a lid for 1 minute. Drizzle additional ranch over melted cheese if desired. Add lettuce and tomato if using. Place croissant tops on and press gently. Serve immediately while hot.