Break the Symphony bars into individual pieces and place them in the slow cooker.
Add the heavy cream, milk, and butter to the slow cooker with the chocolate.
Cover and cook on low for 45-60 minutes, stirring every 15-20 minutes until the chocolate is completely melted and the mixture is smooth.
Stir in the vanilla extract and salt until well combined.
Turn the slow cooker to the warm setting to keep the fondue at the perfect consistency throughout your party.
Serve with strawberries, banana slices, pound cake cubes, pretzels, marshmallows, or graham crackers for dipping.