Stovetop:
Place the cream and butter in a heavy 1-quart saucepan over moderate heat. Stir until the butter is melted and the cream just comes to a low boil. Add both sugars and stir for a few minutes until they are dissolved. (The surest test is to taste; cook and taste until you do not feel any undissolved granules in your mouth.)
Reduce the heat. Add the salt and cocoa and stir briskly with a small wire whisk until smooth. (If the sauce is not smooth - if there are any small lumps of undissolved cocoa- press against them, and stir well, with a rubber spatula.) Remove from the heat.
Microwave:
In a 1 qt pyrex cup (or saucepan, if cooking on stovetop) mix sugars, cocoa, and salt; add cream and mix thoroughly to wet all.
Place butter on top and MW on high until mixture begins to boil.
Stir well, until butter melts into mixture, then MW 2 min. on med-low - 3-4 setting - being sure it doesn't boil over.
Serve immediately or reheat slowly, stirring frequently, in the top of a double boiler over hot water, or in a heavy saucepan over the lowest heat.