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In a small bowl, whisk together cocoa powder, sugar, and salt.
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Pour milk into a slow cooker. Add the cocoa mixture and whisk until the cocoa is dissolved and no lumps remain.
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Add chopped dark chocolate pieces.
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Cover and heat on low for 2 hours, stirring occasionally, until hot and the chocolate is completely melted.
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Stir in vanilla extract just before serving.
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Ladle into mugs and top each with a handful of large marshmallows. If desired, add a dollop of whipped cream.
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Hot chocolate can stay warm in the slow cooker on the 'warm' setting for up to 4 hours. Stir occasionally to prevent a film from forming on top.