Combine sugar and cocoa powder in a saucepan, whisk the water in to make it smooth.
Add salt, and heat over medium heat until boiling, then turn the heat down and simmer for a few minutes.
Remove about a tablespoon of the mixture, and mix with the corn starch.
Blend the corn starch mixture back in, and simmer for a few more minutes until thickened.
Heat, stirring, until the sugar is completely dissolved.
Let simmer gently for 10 minutes until slightly thickened.