Heat a large saucepan over medium-high heat. In a small bowl whisk together the cocoa powder, granulated sugar, and salt. Pour one cup of the milk into the sauce pan and add the cocoa powder and sugar mixture. Whisk until smooth and all of the lumps are removed.Add the remaining milk and whisk together well. Heat the hot chocolate mixture until very warm, but do not boil.Remove the hot chocolate from the heat and add ½ of a bar of milk chocolate. Mix until the chocolate is melted. Add vanilla if desired.Pour the hot chocolate into mugs and enjoy!