Chop up the chocolate bar and place it in a large heat safe bowl. Place a fine mesh sieve over top of the bowl and set aside.
In another medium mixing bowl, add in the egg yolks and starch and whisk until smooth. Set on top of a damp rag on your work surface so it won't slide around.
In large pot add in all of the milk, sugar, heavy cream, salt, cocoa powder, and vanilla. Heat over medium low heat on the stove, whisking frequently, until just beginning to simmer. The cocoa powder and sugar should have dissolved!
Once just beginning to bubble, remove the pot from the heat. While continuously whisking the egg yolks, slowly pour in the hot chocolate milk.
Once it's all been added, pour it back into the pot and heat over low heat (medium low if an electric stove). The mixture will already have started to thicken but you'll just want to cook it for about 5 minutes over low heat ensuring the cornstarch has been cooked out.
Remove the pot from the heat again and pour it through the fine mesh sieve into the large bowl with the chocolate.
Remove the sieve and whisk to combine until smooth. Lay a piece of plastic wrap over the surface and let it come to room temperature.
Once cooled, transfer it to the fridge to cool completely overnight.