Place a skillet over medium heat and add a small knob of butter.
Sauté zucchini rounds in a single layer, seasoning with salt, pepper, and garlic powder until tender and slightly caramelized (approximately 2 minutes per side). Remove from heat.
Spread the remaining butter evenly onto one side of each bread slice.
Place one slice of bread, butter-side down, in the skillet over medium-low heat.
Layer half of the shredded cheese, the sautéed zucchini, and the remaining cheese onto the bread.
Top with the second slice of bread, butter-side up.
Grill the sandwich for 3 to 4 minutes per side, or until the bread is golden brown and the cheese is fully emulsified and melted.