Place the white chocolate in a large heat-proof bowl.
Combine the butter and cream in a small heavy saucepan over medium heat.
Bring the butter and cream to a boil.
Pour the mixture over the white chocolate.
Let the mixture sit for 5 minutes.
Whisk in the lemon juice, zest, and salt until smooth. Keep whisking until the mixture is glossy.
Chill the white chocolate ganache mixture until firm (about 3 hours).
While the ganache is chilling, carefully remove the outer yellow zest from the lemon in thin layers, making sure to avoid having too much white pith attached.
Add the zest pieces to the food processor along with the granulated and sanding sugars.
Process the mixture until there are no large pieces of the zest left.
Transfer the lemon sugar to a shallow bowl and set aside.
Use a 1 ¼-inch cookie scoop or spoon to scoop balls of the chilled ganache onto a plate.
Use your hands to roll the balls smooth. If they become too soft, place the plate in a refrigerator until firm enough to handle.
Once the truffles are rolled, chill them for 30 minutes.
Roll the truffles in the lemon sugar to evenly coat. Keep chilled until ready to serve.