Place the whole milk and heavy cream in a small saucepan over low heat. Heat gently to avoid scalding, stirring occasionally to warm the mixture evenly.

Add the dark chocolate to the warm milk and cream mixture. Whisk continuously over low heat until the chocolate is fully melted and the mixture is smooth and creamy.

Once the chocolate has melted, remove the saucepan from the heat. Stir in the vanilla extract and instant coffee or espresso powder until fully incorporated and dissolved.

Taste the hot chocolate and add sugar gradually according to your preference. Stir well to dissolve the sugar completely.

Pour the rich hot chocolate into small cups. Optionally, top with a generous dollop of whipped cream for an extra luxurious finish. Serve immediately while hot.