Warm a small amount of milk: Measure out about 1/2 cup of the whole milk and warm it slightly in the microwave for 20-30 seconds, just enough to take the chill off.

Whisk cocoa into a paste: Add the cocoa powder to the warm milk and whisk until completely smooth with no dry clumps remaining. This step keeps the final drink from turning gritty.

Combine everything in the slow cooker: Pour the cocoa paste, remaining milk, heavy cream, condensed milk, chocolate chips, vanilla extract and salt into the crock pot. Stir once with a whisk to combine.

Cover and cook on low: Cover the slow cooker and set it to low heat. Cook for 2 hours, which lets the chocolate chips melt fully without scorching the milk.

Stir every 30 minutes: Lift the lid and whisk the mixture every 30 minutes so the melted chocolate doesn't settle as a layer at the bottom of the crock.

Check for glossy texture: After 2 hours the hot chocolate should look glossy and coat the back of a spoon, with thin steam rising but no rolling bubbles.

Switch to warm setting: Turn the slow cooker to the warm setting once it's fully heated through, which keeps it ready to serve for a few hours.

Ladle into mugs: Use a ladle to portion the hot chocolate into mugs, leaving room at the top for toppings.

Top with marshmallows: Add marshmallows to each mug right before handing it over so they stay soft and float on top instead of melting into the pot.