On a heated skillet, add the extra virgin olive oil and heat through. Add the mushrooms and saute for about 3 minutes.
Add the kosher salt to your liking to allow the mushrooms to release moisture and begin to shrink. Increase the heat and saute the mushrooms until browned. Season with black pepper then remove from heat and set aside.
Heat the skillet again on medium-low heat then add truffle oil and heat through. Spread each slice of bread with butter.
Place the slices of bread, buttered side down on the pan then add an equal amount of cheese and the sauteed mushrooms you set aside on top. Layer the second slice of bread on top with the buttered side up.
Cook until the sandwiches’ undersides are golden brown. After about 4 minutes, flip the sandwiches with a spatula to let the other sides brown. Press them down gently for the cheese to melt.
Remove the sandwiches and place them on a cutting board. Cut into halves and serve immediately.