Add the chocolate, condensed milk, and yellow coloring to a microwave-friendly bowl.
Heat in 30 second increments at half speed. Stir well between each increment, until the mixture is creamy and smooth.
Next, stir in the lemon zest, limoncello, and two cups of the confectioner’s sugar. (For an alcohol-free version, use 1/2 tablespoon of lemon juice and 1/2 tablespoon of vanilla extract instead of the limoncello. If you use one full tablespoon of lemon juice, the truffles will be very tart, but still quite delicious!)
Cover and refrigerate for two hours to firm up.
Once chilled, place the remaining one cup of confectioner’s sugar into a shallow bowl. Set aside. Line a baking sheet with parchment paper.
Measure out a heaping teaspoon of the mixture.
Roll into balls and then roll in confectioner’s sugar.
Place on the parchment-lined baking tray.
Once all of the balls are rolled, place the baking tray in the fridge for an hour to set up.
Transfer to a food-safe container with a lid. Keep refrigerated.