Start by gathering all your ingredients. Lay out the sourdough bread, pesto, cheddar cheese, and butter on your cutting board for easy access.
Spread 1 tablespoon of pesto on one side of each slice of bread. Then, layer on 1 slice of cheddar cheese on top of the pesto on each slice.
Heat your skillet over medium heat. Place the two slices of bread together, with the pesto and cheese sides facing each other.
Spread 1 tablespoon of butter on the outside of one slice of bread. Place the sandwich butter-side down into the skillet. Spread the remaining 1 tablespoon of butter on the outside of the other slice.
Cook for about 3-4 minutes, or until the bottom is golden brown and the cheese starts to melt.
Carefully flip the sandwich over to the other side and cook for another 3-4 minutes until golden brown and the cheese is fully melted.
Remove the panini from the skillet and allow it to cool slightly. Cut it in half and serve immediately.
