1. Put bittersweet chocolate and cream into a heatproof bowl set over a pan of simmering water. Stir until smooth; remove from heat. Stir in butter.

2. Pour into a loaf pan. Cover with plastic wrap, and refrigerate until very cold and set but still pliable, about 2 hours.

3. Using a 1 1/4-inch ice cream scoop or a tablespoon, scoop balls of ganache, transferring them to baking sheets lined with parchment paper as you work. Refrigerate 10 minutes.

4. Using your hands, roll each ball into an egg shape. Place in freezer until very firm, about 1 hour.

5. Meanwhile, put 1 1/2 pounds white chocolate into a heatproof bowl set over a pan of simmering water. Stir until smooth; remove from heat. Divide between 2 medium bowls. Add food coloring to 1 bowl until desired blue shade. Add a few drops of food coloring to other bowl, and swirl lightly with a toothpick.

6. Working with one at a time, dip half of the eggs in blue chocolate, then dip remaining eggs in swirled chocolate, letting excess chocolate drip back into bowl. As you work, transfer eggs to a baking sheet lined with parchment paper; refrigerate until set, about 1 hour. Truffles can be refrigerated in an airtight container up to 2 weeks.

7. For sparkly truffles, brush luster dust onto blue eggs to coat if desired.