In a skillet, cook the ground beef over medium heat until browned, about 5 minutes. Drain excess fat.
Add taco seasoning to the beef and stir well. Remove from heat.
Butter one side of each slice of bread. Place four slices, butter side down, on a griddle or skillet over medium heat.
Layer each bread slice with one slice of cheese, then a spoonful of the beef mixture, followed by another slice of cheese. Top with remaining bread slices, butter side up.
Grill the sandwiches until the bread is golden brown and the cheese is melted, about 3-5 minutes per side.
Notes
For a spicier kick, consider adding sliced jalapeños or hot sauce to the beef mixture.