Mix the cocoa powder, sugars, salt, and cornstarch together in a bowl and set aside.

Put the butter, unsweetened chocolate, corn syrup and 1/2 the heavy cream into a medium saucepan and heat over low heat, constantly stirring, until the chocolate has completely melted. Dont worry if it looks like the chocolate sauce has split, it will all be fine in the end!

Pour the sugar mixture into the chocolate sauce, and keep stirring. It will look crumbly like a cookie dough.

Pour the remaining half of the heavy cream into your sauce and keep stirring.

Continue to cook over low heat until you see all the sugar has dissolved. If this is taking too long, add an extra splash of cream to help things along, but not too much if you want a thick sauce.

Finally, remove from the heat and stir in the vanilla. Pour into a pint jar and let it cool on counter uncovered without moving or it could crystalize. If it does, just reheat, dissolve crystals and pour in clean jar.

Notes: The recipe above is my final. Followed as written, produce a hard consistency and crystalized pretty badly. I reheated to dissolve the crystals, added more cream and some of corn syrup. Perfect.