Place white chocolate in a large heatproof bowl. Place the butter on top. Do not mix. Set aside.
Pour heavy cream into a small saucepan and heat over medium heat, whisking occasionally, until it just begins to boil. Remove from heat and whisk in the lemon extract (carefully-- it will bubble up).
Pour the hot mixture over the white chocolate and gently stir with a wooden spoon2 in one direction. Do not stir forcefully.
Once the mixture is completely smooth, cover it with plastic wrap pressed to the surface of the mixture. Allow to sit at room temperature for 1 hour, then transfer to the refrigerator for at least 6 hours (I always chill mine overnight). After chilling, the ganache mixture will be smooth and soft, but still manageable by hand.
When you are ready to roll your truffles, line 2 large baking sheets with parchment paper or silicone baking mats. Pour powdered sugar or sprinkles into a bowl and set aside.
Measure desired amount of ganache based on how big you want the truffles to be (I use a melon baller, but you can also use a teaspoon measure), roll between your hands into a ball3, then roll in the sprinkles. Place on the baking sheet. Continue this process until you have used all of the ganache. Serve immediately or cover tightly and chill in the refrigerator until ready to serve.
Sprinkles: I like tiny non-pareils or sanding sugar for coating.
Wooden spoon: it is crucial to use a wooden spoon as metal or plastic will retain heat and distribute it unevenly.
Rolling truffles: I found it easiest to wipe my hands clean after making each truffle. This may sound tedious, but it helps keep things smooth and clean.