Makes 64 truffles or more
2 large egg yolks, at room temperature
1 pound bittersweet or semisweet chocolate (no more than 62% cacao), coarsely chopped
10 tablespoons (5 ounces) unsalted butter, cut into small pieces
1/8 teaspoon salt, preferably fine sea salt
1/3 cup (1 ounce) unsweetened cocoa powder, preferably natural, or as needed
8-inch square baking pan, lined on the bottom and all sides with foil
Put the egg yolks in a small heatproof bowl, preferably stainless steel, and set the bowl in a large container of very hot water to heat the egg yolks until barely lukewarm. Set aside.
Meanwhile, put the chocolate, butter, and salt in a medium heatproof bowl, preferably stainless steel, set it in a wide skillet of barely simmering water, and stir frequently until the chocolate and butter are melted and the mixture is smooth and quite warm. (If you have an instant-read thermometer, the temperature should be between 120° and 130°F.) Scrape the mixture into the food processor and set the strainer over the processor bowl.
Bring ½ cup water to a boil in a very small saucepan or in a glass measure in the microwave. Remove the egg yolk bowl from the large container and immediately pour the boiling water steadily into the egg yolks, stirring constantly with a silicone spatula. (When the yolks and water are blended, the temperature should be at least 160°F.) Pour the mixture through the strainer into the food processor. Tap the strainer against the bowl to encourage all of the liquid to flow through, but don’t press on or mess with any bits of cooked egg in the strainer.Process the mixture for a few seconds, then scrape the bowl and process again for 20 or 30 seconds, or as long as it takes for the mixture to thicken and resemble satiny-smooth chocolate pudding. Scrape the mixture into the lined pan and spread it evenly. Let cool, then cover and refrigerate until firm, at least a few hours.
To shape the truffles: Put half of the cocoa in a small bowl. Remove the baking pan from the refrigerator and use the liner to remove the truffle sheet. Invert it on a cutting board and peel off the liner. Cut the truffles into 1-inch squares (or smaller, if you prefer) and toss them in the cocoa powder, adding more cocoa as necessary. You can leave the truffles square or dust your hands with cocoa and roll them into balls. Shake the truffles gently in a coarse strainer to remove excess cocoa. Store the truffles, tightly covered, in the refrigerator for up to 2 weeks, or in the freezer for up to 3 months.
Remove the truffles from the refrigerator about 15 minutes before serving. Sift a little extra cocoa over them as necessary.