Pound the chicken breasts to an even thickness (about 3/4 inch) using a meat mallet or rolling pin. Pat dry with paper towels. Season both sides with 1/2 teaspoon salt, 1/2 teaspoon black pepper, and 1 teaspoon garlic powder. Heat 1 tablespoon olive oil in a large skillet over medium-high heat. When shimmering, add the breasts and cook 4–6 minutes per side until an instant-read thermometer reads 165°F (74°C). Remove to a cutting board and let rest 5 minutes, then slice into 1/4-inch thick pieces.

Cook bacon

While the chicken rests, cook the bacon. In a separate large skillet or in the oven on a rimmed baking sheet at 400°F (200°C) for 12–18 minutes, cook the 8 bacon strips until crisp. Transfer to paper towels to drain and cool, then cut each strip in half if desired.

Make sauce

In a small bowl combine 1 cup mayonnaise and 2 teaspoons Dijon mustard. Season with a pinch (about 1/8 tsp) salt and a grind of black pepper. Stir until smooth. Taste and adjust—add more mustard for tang or a pinch of sugar if you prefer a slightly sweeter mayo.

Toast bread

Spread the 2 tablespoons softened butter evenly on one side of each of the 8 bread slices. Heat a large skillet or griddle over medium heat. Toast the buttered sides down in batches until golden brown, about 2 minutes per side, pressing lightly so they brown evenly. Alternatively, toast under a broiler or in a toaster oven, then spread butter on the warm slices.

Assemble sandwiches

Lay out 2 slices of toasted bread per sandwich (you should have 4 sandwiches). Spread about 1 tablespoon of the mayonnaise mixture on the unbuttered side of each bottom slice. On the bottom slice place: a layer of lettuce (2 leaves), 2–3 slices of cooked chicken, 1 slice of cheese, 2 tomato slices (season tomatoes lightly with a pinch of salt), and 2 pieces of bacon. If using avocado, add 2–3 slices now. Add the middle slice of toast (this becomes a double-decker club): spread 1 tablespoon mayo mixture on the top of the middle slice, then repeat the layering: lettuce, remaining chicken slices, remaining bacon, cheese (or omit if you prefer only one cheese layer). Spread mayonnaise on the top slice of bread and place mayo-side down to finish.

Finish and serve

Press each sandwich gently, then insert 2 long toothpicks (one near each end) or skewer through the center to hold the layers. Using a sharp knife, cut each sandwich diagonally into quarters (or halves if preferred). Serve immediately with chips, fries, or a pickle.