In a medium-heavy saucepan, combine the cocoa, chocolate, salt and milk. Place the mixture over medium heat, whisking gently. Once the chocolate melts and cocoa dissolves, raise the heat to medium-high; whisk more vigorously to form froth on surface. When mixture bubbles around edges and seems ready to boil, remove from heat. Do not let it boil.
Ladle into 4 small cups with some froth on each. Serve immediately.
• For a richer version of the hot chocolate, whisk 1/2 cup chilled heavy cream with 1 teaspoon sweetened cocoa until it forms peaks. Spoon dollops of chocolate whipped cream on top of hot chocolate before serving and sprinkle with grated chocolate.
• Add a touch of peppermint extract to the milk after the chocolate has melted; garnish with crushed peppermint candy.