Combine the chopped white chocolate and heavy cream in a heatproof bowl. Gently melt the mixture using a microwave in 20-second bursts, stirring well after each, or alternatively, set the bowl over a saucepan of simmering water and stir occasionally until smooth. Once fully melted, blend in the softened butter until the mixture is unified. Mix in the lemon zest from three lemons along with the fresh lemon juice. Cover and refrigerate for no less than three hours, or until the mixture becomes firm.

In a small bowl, blend the sifted powdered sugar with the zest of half a lemon, using your fingers or a fork to combine them thoroughly. Remove the chilled chocolate mixture from the refrigerator. With a melon baller, small scoop, or teaspoon, portion out small amounts and shape them into balls by hand (each about 12g or 1 inch in diameter). If they soften, return to the refrigerator to firm up. Roll each formed truffle in the lemon-sugar coating and place on a parchment-lined baking sheet. Chill again until they are set.