Perfect Chocolate Truffles

1. FOR THE GANACHE: Melt chocolate in medium heatproof bowl set over pan of almost-simmering water, stirring once or twice, until smooth. Set bowl aside. Bring cream, butter, and corn syrup to simmer (about 160 degrees) in nonreactive pan over low heat. Remove pan from heat, cool for 5 minutes, then whisk into chocolate. Whisk in cognac. Refrigerate mixture until it cools to 80 degrees, 15 to 20 minutes.
2. Using stand mixer fitted with whisk, whip mixture at medium speed until thickened to a texture like store-bought canned chocolate frosting, 25 to 30 seconds. Spoon ganache into large pastry bag fitted with 1⁄2-inch plain tube. Hold bag perpendicular to pan and with tip about 3⁄4 inch above work surface, pipe 3⁄4-inch mounds (pulling tube away to the side to avoid leaving points) onto parchment-lined baking sheet. Alternatively, scoop mounds with tiny (less than 1 tablespoon) ice cream scoop or melon baller. Refrigerate mounds until hardened, at least an hour.
3. FOR THE COATING: Melt chocolate in heatproof bowl over almost-simmering water, then cool to 90 degrees. Arrange chilled truffle mounds, bowl of melted chocolate, and high-sided roasting pan filled with cocoa on work surface. Working one mound at a time, dip palm of one hand about 1⁄4-inch deep into melted chocolate, pass one truffle mound with other hand to chocolate-covered hand and close hand around mound to coat, re-dipping hand into chocolate every third or fourth mound. Drop coated truffle into cocoa; roll to coat using fork held in now empty clean hand, leaving truffles in cocoa until chocolate coating has set, about 1 minute. Repeat process until all mounds are in pan of cocoa. Gently roll 5 to 6 truffles at a time in medium strainer to remove excess cocoa, then transfer to serving plate or tightly covered container. (Truffles can be refrigerated for up to 1 week.)

CHOCOLATE NUT TRUFFLES
Pulse 1 cup unsalted pistachios, almonds, pecans, or walnuts with 1 tablespoon sugar to a fine powder (about the texture of coarse sand) in food processor, 45 to 60 seconds. Substitute ground nuts for cocoa.

CHOCOLATE COCONUT TRUFFLES
Adjust oven rack to middle position and heat oven to 350 degrees. Spread 1 cup shredded, sweetened coconut in even layer on baking sheet; toast until golden brown, 4 to 5 minutes, stirring every 2 minutes. Substitute toasted coconut for cocoa.