Use a metric ton of chocolate. Typical hot chocolate recipes have about an ounce of chocolate and a spoon or two of cocoa to a cup of milk, but for max mouthfeel you want to use at least 3 times that amount of chocolate. Some recipes even add a cornstarch slurry to encourage thickening.

Look up Italian or French hot chocolate recipes. I drink this stuff in (double) espresso cup portions because it's so gloriously rich.