Water Ganache

Line a 9- by 13-inch rimmed baking sheet with parchment; set aside.

In a medium saucepan, bring water, salt, and corn syrup (if using) to a boil. Remove from heat, add chocolate, and whisk until smooth, thick, and glossy. Set aside, using a flexible spatula to stir gently and frequently, until ganache registers 94ºF (34ºC), about 20 minutes. If using ganache to frost a cake, use immediately. If using ganache to make truffles, scrape ganache into prepared baking sheet; press plastic wrap or parchment against surface of chocolate and let sit at cool room temperature until ganache is set, at least 8 hours and up to 48 hours.

For the Truffles - Using a fine-mesh sieve, sift cocoa powder onto a 13- by 18-inch rimmed baking sheet.

For the Truffles - Bring a medium saucepan to a gentle simmer over medium heat. Reduce heat to low to keep water at a bare simmer. Place chocolate in a medium heatproof bowl, set it over the saucepan and, stirring occasionally with a flexible heatproof spatula, cook until chocolate is melted and smooth. Remove bowl and set on a heatproof work surface, stirring occasionally, until cooled but still fluid.

For the Truffles - Meanwhile: Using an offset spatula or butter knife, cut ganache into 12 rows lengthwise and then 9 crosswise to form 108 evenly sized squares. Using your hands and working one at a time, roll each square into a round ball. Set balls aside in a large bowl, on a large plate, or a 9- by 13-inch rimmed baking sheet while you shape the remaining truffle balls.

For the Truffles - Working one at a time, drop truffle into bowl of melted chocolate and, using a slotted spoon or spatula, toss truffle to evenly coat with chocolate. Use slotted spoon to remove truffle, allowing any excess chocolate to drip off, and place in cocoa powder. Gently shake baking sheet after each addition to roll truffles so they are evenly coated with cocoa powder; let sit at cool room temperature until firm, about 1 hour. Repeat with remaining truffles.

For the Truffles - When truffles have solidified, place 5 to 7 truffles in a large fine-mesh sieve. Shake to remove excess cocoa; set aside. Repeat with remaining truffles.