Heat a large skillet over medium-high heat. Add the ground beef and cook, breaking it up with a spatula, until no pink remains, about 6 to 8 minutes. Drain excess fat if needed.

Stir in the taco seasoning and water. Reduce heat to medium-low and simmer for 3 to 4 minutes until the mixture thickens and looks saucy but not wet. Remove from heat.

Lay 4 tortillas on a clean surface. Divide the beef mixture evenly among them, spreading to within 1 inch of the edges. Top each with 3/4 cup cheese, then press the remaining tortillas on top to seal.

Wipe out the skillet and return to medium heat. Brush the top of each quesadilla lightly with softened butter.

Place 2 quesadillas in the skillet, buttered side down. Cook for 3 to 4 minutes until the bottom is deep golden brown and crispy. Brush the tops with butter, flip carefully, and cook another 3 to 4 minutes. Transfer to a cutting board and repeat with remaining quesadillas.

Let rest for 2 minutes, then cut each quesadilla into 4 wedges. Serve hot with salsa, sour cream, and cilantro on the side.