In heavy-bottomed, medium-size saucepan, heat heavy cream until bubbles begin to appear around edge of pan. Place chopped chocolate in medium-size bowl. Pour half of hot cream over chocolate; let stand 30 seconds for chocolate to melt. Slowly whisk until smooth and emulsified. (If ganache separates and becomes very liquid, use a handheld immersion blender to emulsify.) Gradually add remaining hot cream, slowly whisking until all the cream is incorporated and the ganache is smooth and emulsified. (If ganache separates at this point, add small amount of cold cream and whisk well.) Ganache should be thick, shiny and smooth. Add Grand Marnier. Pour ganache onto plastic wrap-covered baking sheet; spread evenly with rubber spatula. Cover with plastic wrap; let stand until cooled, at least 4 hours (it will thicken and set).
When ganache has cooled to consistency of toothpaste, scrape into pastry bag fitted with 1/2-inch plain tip; do not stir ganache. Pipe 1-inch diameter mounds, spacing 1 inch apart, on parchment paper-covered baking sheet (you can also use a spoon). Let truffles harden at room temperature for a couple of hours (or in refrigerator for 15 minutes), until they are hard enough to roll with your hands.
To roll truffles, place mound between palms, squeeze slightly, and roll. (You can wear surgical gloves. Otherwise, make sure hands are clean.) If too soft, refrigerate 2 hours, until firm.
Place three-quarters of the bittersweet chocolate in a microwave-safe bowl. Microwave on HIGH power in 30-second increments until chocolate is nearly melted. Do not overheat; chocolate should not look melted, but rather it should retain its shape; lumps will slowly melt from residual heat of the chocolate. Gradually add the small pieces of the remaining quarter of the chocolate, using a hand-held immersion blender to mix together. Repeat with white chocolate.
Have tempered chocolate ready in 2 separate bowls. Drop truffle into one of the bowls of tempered chocolate; retrieve the truffle with a plastic dipping fork. Let the excess chocolate drip back into the bowl. Gently scrape the bottom of the truffle against side of bowl to remove any excess chocolate. Immediately roll dipped truffle in desired garnish. Place on parchment paper-covered baking sheet. (At this stage it is good to have a friend help because it is hard to dip and roll at the same time.) Repeat using all truffles, tempered coatings and garnishes.
Let truffles stand until set, about 5 minutes. Truffles can be stored airtight at room temperature for up to 2 weeks.