Melt butter in the microwave or in a small pot.
Dice mushrooms and place in a large, dry frypan on medium heat. Cook for about 5 minutes, until golden, then remove from heat. Season well with salt and pepper.
Slice bread into 8 pieces to make 4 sandwiches. Spread each piece of bread with truffle cream sauce. In each sandwich, place a small amount of grated cheese, 1/4 cup of cooked mushrooms and a little more cheese before adding the top piece of bread.
Heat a large nonstick frypan on low-medium heat. Use a pastry brush to brush the pan with melted butter. Place a sandwich into the pan, and brush the top piece of bread with butter too. Cover frypan with lid and cook for about 3 minutes on each side. Repeat with remaining sandwiches and enjoy!