Shred cheeses with the shredding disk into food processor work bowl. Stir in cornstarch until cheese is completely coated.
Heat beer in medium saucepan on medium heat, until just simmering. Reduce heat to medium low. Add cheese, stirring slowly, until smooth. Remove from heat; stir in butter. Thin fondue with stock if needed.
Serve with toasted pretzel rolls.
Cook's Tip: Fondue can be served immediately or refrigerated up to 24 hours. Reheat in microwave at 50% power, stirring every 15 seconds.