Remove labels from the condensed milk cans (keep cans sealed).
Place the cans in a slow cooker and cover completely with water (at least 2–3 inches above the cans).
Cover and cook on low for 7–8 hours for a thick caramel consistency.
Ensure the cans remain fully submerged during cooking (add water if needed).
Turn off heat and allow the cans to cool completely before opening.
Carefully open the cans and transfer the caramel to a bowl.
Stir in vanilla extract, salt, or butter if desired for extra flavor.
Mix until smooth and creamy, then store or serve.