Coat a 9-by-5-inch loaf pan with cooking spray; line with plastic wrap, allowing a 2-inch overhang; set aside. Put chocolate and butter into a medium heatproof bowl set over a pan of simmering water; stir until chocolate and butter have melted. Remove from heat.Advertisement
Bring cream, corn syrup, and brown sugar to a boil in a small saucepan over medium-high heat, stirring until sugar has dissolved. Immediately pour over chocolate mixture; whisk until smooth. Whisk in yolks, whiskey, coffee extract, and salt. Pour into prepared pan. Cover with plastic wrap, pressing it directly on surface to prevent a skin from forming. Refrigerate until firm enough to roll into balls, about 4 hours (up to 1 week).
Using a 1-inch ice cream scoop or a medium melon baller, scoop chocolate into balls, immersing scoop completely, then rotating it; transfer to a rimmed baking sheet lined with parchment. Refrigerate until firm, about 15 minutes.
Put cocoa powder into a shallow dish. Remove truffles from refrigerator. Roll each ball in your palms until completely smooth, then roll in cocoa powder to coat (if truffles become too soft to roll, return to refrigerator until firm); transfer to a rimmed baking sheet lined with parchment paper. Cover with plastic wrap, and refrigerate at least 30 minutes (up to 3 days).
To help cut through the ganache when forming truffles, first dip your scoop into hot water, then dry it with a towel.