In a medium bowl, whisk together eggs until combined. Add salt and pepper and 1/4 cup grated cheese. Heat a medium nonstick skillet over medium-high heat, and add butter. When butter has melted, add the egg mixture, and cook about 30 seconds. Add salsa to pan, slowly stirring the egg mixture to scramble it as it cooks, about 1 minute. Immediately remove from heat.
Fill taco shells with egg mixture, and top with the remaining cheese, lettuce, sour cream, and tomatoes. Serve with tortilla chips or black beans, if desired.