Lightly butter one side of each tortilla. Return the skillet to the stove over medium-high heat. Place one tortilla buttered side down in the skillet. Layer half of the cheese, the spinach, and the remaining cheese over the tortilla. Top with the remaining tortilla.
When the cheese is melting and the tortilla facing the skillet begins to brown, use a large spatula to carefully flip the tortilla over and cook on the second side for a few minutes.
When everything has melted together and the tortillas are golden and crisp, remove the quesadilla from the skillet and slice it into wedges. Serve warm with hot sauce.