Combine water and cocoa powder in a large pot, whisk to blend. Bring to a boil, whisking to blend and bloom the cocoa powder. Add milk, sugar and kosher salt and bring to a simmer, then remove from heat.
Add chocolate and whisk to combine until the chocolate is melted.
When ready to serve, in a medium bowl with a whisk (or hand mixer/stand mixer), whip the heavy cream until you have nice soft peaks. No need to season this with sugar, but if you want, a little powdered sugar is the way to go. Top hot chocolate with an unmodest amount of whipped cream.
DO AHEAD: Hot chocolate can be made and chilled 5 days ahead. Rewarm gently before serving, thinning with milk as needed.
