Put the semisweet chocolate, bittersweet chocolate, cream, milk, vanilla and salt in a 6-cup heatproof glass bowl that will fit completely inside a 3- to 4-quart slow cooker. Cover and cook on low for 45 minutes.
Uncover and stir the chocolate mixture until smooth. Stir in the bourbon or liqueur if using. Continue to cook, covered, until the chocolate is completely melted and warm, 15 to 30 minutes. Whisk the mixture until smooth. Switch the slow cooker to the warm setting and either spoon the chocolate mixture into small serving bowls or serve it right out of the slow cooker with assorted dippers on the side.
While keeping the fondue warm, water will collect on the inside of the lid of the slow cooker; when you remove the lid to stir the fondue, wipe the excess water off as too much water dripping into the chocolate will eventually cause it to seize or separate.
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