In a bowl, mash the ripe avocado and mix in mayonnaise, garlic powder, onion powder, salt, and pepper.
Spread butter on one side of each slice of bread. On the unbuttered side, layer the avocado mixture, mozzarella cheese, and cheddar cheese. Top with another slice of bread, buttered side up.
Heat a skillet over medium heat. Cook the sandwiches for about 3-4 minutes on each side or until golden brown and the cheese is melted.
Remove from the skillet, cut in half, and serve. Garnish with fresh basil leaves if desired.
Notes
You can add other ingredients like tomatoes or spinach for more flavor and nutrition.