You can use extra-sharp cheddar for these burgers if you prefer your cheese not to soften as much. However, I recommend using a mild or sharp cheddar for this recipe. Mild cheddar is aged for a few months and has a creamy texture, while sharp cheddar is aged longer and has a bolder flavor but still softens. You can choose either yellow or white cheddar based on your preference for color. In case you don't have slices, you can sprinkle shredded cheddar on your burger too!