Combine the mayonnaise, sour cream, buttermilk, garlic and onion powders, chives, salt, and pepper. Mix well and refrigerate, covered, until ready to serve.
Make the burgers:
Prepare the grill for direct heat and preheat to medium-high.
Combine the ground chuck and brisket, making sure to mix well. Divide into 6 equal portions. Form each portion loosely into a ¾-inch thick patty and make a depression in the center of each patty with your thumb. Brush the patties with oil and season generously with salt and pepper.
Grill the burgers 3 – 4 minutes per side for medium rare. Place the hamburger buns, cut side down, on the grill and cook for 10 – 20 seconds to lightly toast.
Spread a little of the ranch dressing on the bottom buns, top with lettuce, the burgers, and tomato slices. Drizzle generously with more ranch dressing and top with the top of the bun. Serve any remaining dressing on the side