Start by gathering all your ingredients. Make sure your butter is at room temperature for easy spreading, and have your cheese slices ready to go.
On one side of each slice of bread, spread 1 teaspoon of mayo evenly. This will add flavor and help achieve a crispy crust.
Flip the slices of bread over so the mayo side is facing down. Layer the cheeses on one slice: place the sharp Cheddar, followed by the Gouda, and finish with the Muenster.
Sprinkle the garlic powder and salt evenly over the cheese layer. This is where the magic happens, adding depth to your sandwich.
Place the second slice of bread on top, mayo side up.
In a non-stick skillet over medium heat, add the room-temperature butter. Allow it to melt, coating the bottom of the pan.
Carefully place the sandwich in the skillet. Cook for about 3-4 minutes, or until the bottom is golden brown.
Using a spatula, gently flip the sandwich. Cook for an additional 3-4 minutes on the other side, allowing the cheese to melt completely and the bread to crisp up.
Once both sides are golden and the cheese is gooey, remove the sandwich from the skillet. Let it sit for a minute, then slice it in half and serve with your favorite dipping sauce.