Place six bread slices on a clean work surface.
Divide the shredded cheddar and mozzarella evenly over the bread, keeping the cheese slightly away from the edges.
Top with the remaining bread slices and press gently.
Mix the softened butter with the garlic powder, if using.
Spread a thin layer of butter over the top of each sandwich.
Heat a large skillet or griddle over medium-low heat.
Place the sandwiches in the skillet with the buttered side facing down.
Spread butter over the second side while the first side cooks.
Cook for three to four minutes, or until the bottoms are evenly golden.
Turn the sandwiches and cook for another three to four minutes, or until the second side is golden and the cheese has melted.
Transfer the sandwiches to a cutting board and rest for two minutes.
Cut each sandwich into four equal squares or triangles.
Serve warm with tomato soup or marinara sauce.