5 ounces unsweetened chocolate, chopped
1 ounce bittersweet chocolate, chopped (70% cocoa mass would be the ideal)
3/4 cup heavy cream
1/4 cup water
1 1/3 cups granulated sugar
1 tablespoon natural cocoa powder
1/3 cup light corn syrup
1/4 teaspoon salt, preferably flaky sea salt
1 1/4 teaspoons vanilla extract
3 tablespoons cool unsalted butter, cut into teaspoon-size chunks
Place the unsweetened chocolate, bittersweet chocolate, heavy cream, and water in a heavy, medium-size saucepan (preferably enameled cast iron). For the best results, the diameter of the pan should measure about 6 inches in diameter.
Place the pan over low heat to melt the chocolate in the two liquids. Stir the contents of the pan from time to time, using a wooden spoon. In the meantime, thoroughly whisk the granulated sugar and cocoa powder in a small mixing bowl.
Stir in the sugar-cocoa mixture, corn syrup, and salt. Let the mixture warm, continuing on low heat, for 5 minutes, stirring occasionally (by this time the granules of sugar will have dissolved). Raise the heat to moderately low and bring the mixture to a simmer, whisking rapidly. Simmer the sauce for 1 minute, whisking all the while.
Remove the pan from the heat. Whisk in the vanilla extract and chunks of butter.
Strain the sauce though a fine-mesh sieve into a heatproof bowl.
Use the sauce immediately, or cool completely, transfer to a clean storage container, cover tightly, and refrigerate. Gently rewarm the sauce before serving.
up to one week, refrigerated in a tightly-sealed container
◊ when chopping chocolate, be sure to add all of those flakes and shards to the pan
◊ using sea salt adds a clean, clear dimension and flavor to the finished sauce
◊ whisking the butter as a finishing ingredient adds a lovely luster and consistency to the sauce