Lightly butter both sides of the bread. Evenly layer the slices of avocado on one slice of bread. Top with shredded cheese then sprinkle over cayenne pepper and cilantro if desired. Cover with remaining sourdough slice.
Heat a large non-stick skillet over medium. Fry sandwich 2-3 minutes per side or until golden and crispy. Rest 1 minute before cutting in half. Serve with salsa on the side for dipping.