Preheat a skillet over medium heat.
In a small bowl, mash the avocado with a fork until smooth. Season with salt and pepper to taste.
In another bowl, mix the shredded mozzarella and cheddar cheese.
Spread butter on one side of each bread slice. On the other side of two slices, spread a thin layer of mayonnaise.
Place the two slices of bread, mayonnaise side down, in the skillet.
Evenly spread the mashed avocado on the bread in the skillet, then sprinkle with garlic powder and onion powder.
Add a layer of the cheese mixture over the avocado and top with fresh basil leaves if desired.

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Place the remaining two slices of bread on top, butter side up.
Grill the sandwiches until the bread is golden brown and the cheese is melted, about 3-4 minutes per side.
Remove from the skillet and let cool slightly before slicing and serving.
Prep Time: 10 minutes | Cooking Time: 10 minutes | Total Time: 20 minutes
Kcal: 350 kcal | Servings: 2 servings