In a small bowl, combine the butter, garlic, parsley, and salt.
Cut the croissants open and brush the insides with some garlic butter. Toast or broil until golden.
Divide the beef into 8 balls. Heat a flat top grill or cast iron pan until very hot. Place the burger balls down and smash them flat with a heavy spatula or weight. Season with salt and pepper. Cook for 2 minutes, flip, and add a slice of cheese to the top.
Combine the burger sauce ingredients in a bowl. Brush on the top and bottom of the croissants. Place lettuce and tomato on the bottom, and then two stacked burger patties. Close it up, and brush the tops of the croissants with more garlic butter. Enjoy!