Whisk in cocoa powder and salt until no dry pockets remain.
Add sweetened condensed milk and chocolate chips; stir to combine.
Cover and cook on LOW for 2 hours, stirring every 30 minutes to melt the chocolate evenly.
Stir in vanilla. Taste and add sugar if you prefer it sweeter.
Switch to WARM for serving. Ladle into mugs and top with marshmallows.
Notes
Variation ideas: Stir in a little peppermint extract for a minty twist, or add a shot of espresso for mocha. For dairy-free, use canned full-fat coconut milk and a dairy-free dark chocolate.Serving/storage: Keep on WARM for up to 2 hours. Refrigerate leftovers up to 3 days and reheat gently on the stovetop or in the slow cooker on LOW.This recipe is an original creation inspired by classic Easy Slow Cooker Hot Chocolate flavors. All ingredient ratios and instructions are independently developed.