Slice your croissants in half horizontally, creating top and bottom pieces. Shred the mozzarella and cheddar cheeses if not already shredded—freshly shredded cheese melts much more evenly than pre-packaged varieties. I find that a box grater works perfectly for this task and takes just a minute.

Divide the shredded cheese between the bottom halves of both croissants, distributing it evenly. Close the sandwiches by placing the top halves over the cheese. Spread about three-quarters of the butter on the outside of the top croissant halves, and the remaining butter on the outside of the bottom halves. Sprinkle the garlic powder evenly over the buttered exterior of both sandwiches—this adds a subtle savory depth without overpowering the delicate croissant.

Heat a skillet or griddle over medium-low heat. Once the pan is warm, carefully place both buttered croissants onto the surface. Cook for 3-4 minutes without moving them until the bottom is deeply golden and crispy. Flip gently and cook for another 2-3 minutes until the second side is equally golden and the cheese inside is completely melted. I like to press down gently with a spatula during the last minute of cooking to help compress the sandwich slightly and ensure the cheese melts evenly.

Transfer the grilled cheese croissants to a plate and let them cool for just 1-2 minutes before serving. The croissants will continue to crisp slightly as they cool, and this brief rest prevents the molten cheese from burning your mouth.