In a bowl, combine olive oil, garlic powder, paprika, salt, and pepper. Add chicken breasts and toss to coat evenly. Let marinate for at least 10 minutes.
Preheat a grill pan or outdoor grill to medium-high heat. Cook chicken breasts for 6-8 minutes per side, or until internal temperature reaches 165°F (74°C). Let rest for 5 minutes before slicing.
Butter one side of each slice of bread and toast in a skillet or on the grill until golden brown.
Assemble the sandwiches: Spread mayonnaise on the un-toasted side of the bread. Top with lettuce, sliced chicken, and tomato slices.
Place the top slice of bread on each sandwich and serve immediately.
Notes
For an extra touch, you can add a slice of cheese to the chicken while it's still warm and let it melt.