Heat griddle to 350°F or heat 12-inch skillet over medium heat. Top 2 slices of the bread with cheese; cover with remaining bread.
In small bowl, mix butter and taco seasoning mix. Spread half of mixture on one side of each sandwich.
Place sandwiches, buttered sides down, on hot griddle. Spread top sides with remaining butter mixture. Cook 5 to 7 minutes, turning once, until golden brown on both sides and cheese is melted. Serve with salsa as a dip.