Spread 3 tablespoons butter on 1 side of each slice of bread. Place bread, butter side down, in a large cast-iron skillet or electric griddle over medium-low heat until golden brown, 2-3 minutes; remove. In a small bowl, combine cheddar, Monterey Jack and Gruyere. In another bowl, mix together remaining 3 tablespoons butter, mayonnaise, Manchego cheese and onion powder., To assemble sandwiches, top toasted side of 4 bread slices with pepperoni; add sliced Brie. Sprinkle cheddar cheese mixture evenly over Brie. Top with remaining bread slices, toasted side facing inward. Spread the butter-mayonnaise mixture on the outsides of each sandwich. Place in same skillet and cook until golden brown and cheese is melted, 5-6 minutes on each side. Serve immediately.