Lightly grease the slow cooker with nonstick cooking spray.
Add the chocolate chips, heavy cream, milk, butter, vanilla, and salt to the slow cooker. Gently stir to combine.
Cover and cook on low for 1 hour, stirring every 15-20 minutes, until the chocolate is completely melted and smooth.
Once the chocolate is fully melted, whisk the fondue until glossy and smooth. If you prefer a thinner consistency, whisk in additional heavy cream or milk a little at a time until the desired consistency is reached.
Switch the slow cooker to the warm setting and serve the chocolate fondue immediately with your favorite dippers.