Heat the Cheddar cheese and the milk in a microwave-safe bowl, loosely covered, at full power for 1 – 1 ½ minutes (depending on your microwave’s power), stirring at 30 second intervals, until smooth. (Alternatively, in a small saucepan over low heat, whisk together the cheese and milk until smooth, 2 – 3 minutes.)
Stir in the salsa, if desired, and serve it immediately with tortilla chips. (Reheat any leftovers in the microwave or in a saucepan over low heat.)