Step 1: Cook the Chicken

1. Preheat a skillet or grill pan over medium heat.
2. Rub the chicken breasts with olive oil and season both sides with salt, pepper, smoked paprika, and garlic powder.
3. Place the chicken in the skillet and cook for 5-7 minutes per side, or until the internal temperature reaches 165°F (74°C). Remove from the heat and let rest for 5 minutes.
4. Slice the chicken into thin strips or shred it for the sandwich.

Step 2: Prepare the Avocado Spread

1. In a small bowl, mash the avocados with a fork until smooth but slightly chunky.
2. Add the lemon juice and a pinch of salt to enhance the flavor and prevent browning.

Step 3: Assemble the Sandwiches

1. Spread a thin layer of mayonnaise on one side of each slice of bread.
2. On four slices of bread, spread a generous layer of the mashed avocado on the opposite side.
3. Layer the sliced or shredded chicken on top of the avocado.
4. Add a slice of cheddar cheese, followed by a slice of tomato and a lettuce leaf, if using.
5. Top with the remaining slices of bread, mayonnaise side facing out.

Step 4: Grill the Sandwiches

1. Heat a large skillet or griddle over medium heat and add 1 tablespoon of butter.
2. Place the sandwiches in the skillet, working in batches if necessary, and cook for 2-3 minutes per side, or until the bread is golden brown and the cheese has melted. Add more butter as needed for additional batches.

Step 5: Serve and Enjoy

Cut the sandwiches in half and serve immediately. Pair with a side of sweet potato fries, a fresh salad, or a cup of soup for a complete meal.