To prepare ganache, place semisweet chocolate in a large heatproof mixing bowl. Set aside. Combine heavy cream, corn syrup, and orange zest in a medium saucepan. Heat over medium-high heat until mixture boils. Immediately pour mixture through a fine-mesh strainer over chocolate. Stir chocolate mixture with a silicone spatula until thoroughly combined and shiny. Stir in butter, Grand Marnier, and orange oil. Pour mixture into a shallow baking dish or a small rimmed baking sheet and chill overnight.

Use a melon ball scoop warmed briefly in hot water to scoop balls of ganache. Take care to thoroughly wipe off all excess water from the scoop to avoid adding water to the ganache. Quickly roll ganache between your palms to form the truffles into a ball. Place a small amount of melted bittersweet chocolate in the center of your palm and roll truffle to coat. Roll the coated truffle onto a parchment-lined sheet tray. Once the chocolate sets, coat the truffles a second time. Repeat process with remaining truffles.

These rich bites get a double dose of chocolate from semisweet and bittersweet varieties. Make the ganache for the centers of the truffles the day before to allow the mixture to chill and set properly. Finished truffles can be stored in the refrigerator in a tightly sealed container for 1 to 2 days.