Stir sugar, corn syrup, water, salt, and lemon juice together in a microwave-safe measuring cup.
Microwave until mixture reaches 320 to 330 degrees, about 5 to 8 minutes.
Let caramel sit for 5 minutes to darken.
Stir in hot cream a few tablespoons at a time, then add butter.
Serve immediately or refrigerate for later use.
