3 ounces fine-quality unsweetened chocolate, chopped
2 ounces fine-quality 70% bittersweet chocolate, chopped
3 tablespoons water
3/4 cup sugar
1 1/2 cups heavy cream, heated
1 1/2 tablespoons unsalted butter, at room temperature
1 teaspoon pure vanilla extract
3/4 teaspoon kosher salt (or more to taste)
In a food processor fitted with the metal blade, process the chocolate until very fine.
Spray a heatproof 2 cup measuring cup with cooking spray and set aside.
To make the caramel, stir together the sugar and water in a medium heavy saucepan until the sugar is moistened. Heat on medium-high, stirring constantly, until the sugar dissolves and the syrup is bubbling. Stop stirring and allow the syrup to boil undisturbed until it turns deep amber (370 degrees Fahrenheit on an instant read thermometer.) Remove it from the heat and slowly and carefully stir in the hot cream. It will bubble up furiously.
Use a silicone spatula or wooden spoon to stir the mixture gently, scraping down to the bottom of the pan. Return the pan to very low heat, continuing to stir gently for 1 minute, or until the mixture is uniform in color and the caramel is fully dissolved. Remove pan from the heat and gently stir in the butter until incorporated. Pour into prepared measuring cup.
With the food processor running, pour the hot caramel sauce onto the grated chocolate. By the time all of the caramel has been added, the chocolate will be melted and the mixture will be smooth and shiny. Add the vanilla and the salt and pulse 2-3 times.
Scrape ganache into a bowl and let stand uncovered for 1 hour then cover with plastic wrap and let sit at room temperature for two more hours, until firm enough to spread as a filling and frosting.
Makes about 2 cups.