Whisk together the sugar and cocoa powder in a 6-quart slow cooker. Turn the cooker to high, and slowly whisk in the milk to prevent lumps from forming. Add the vanilla and salt, cover and cook for 1 1/2 hours. The sugar and cocoa powder should be completely dissolved.
Whisk in the chopped chocolate a little at a time until it is all incorporated. Replace the lid, and cook until thickened, 30 minutes more.
Turn the cooker to warm, and serve with small bowls of the peppermint candies and mini marshmallows and a small pitcher of schnapps, if using. For minty cocoa, let a few teaspoons of the peppermints melt in an 8-ounce cup or add 2 tablespoons of schnapps.
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