Step 1: In a small bowl, whisk together the cocoa powder, granulated sugar, and cornstarch until thoroughly combined and lump-free.
Step 2: Pour the whole milk into a medium saucepan. Place the saucepan over medium heat and warm the milk gently, stirring occasionally, until it just begins to steam. Do not bring it to a rolling boil.
Step 3: Gradually add the prepared dry cocoa mixture into the warm milk, whisking continuously until it is completely dissolved and smooth. Then, add the finely chopped dark chocolate, continuing to whisk until the chocolate is fully melted and beautifully incorporated into a rich, uniform liquid.
Step 4: Stir in the pinch of salt and the vanilla extract. Reduce the heat to low and continue to gently simmer, whisking constantly, until the hot chocolate reaches your desired luxurious thickness. Taste and adjust sweetness if needed.
Step 5: Once the hot chocolate has achieved a perfect, velvety consistency, carefully pour it into your favorite mugs. Top with a generous dollop of whipped cream, a few marshmallows, or chocolate shavings if desired. Serve immediately and enjoy.