Using a vegetable peeler, remove the zest from the lemons and place in the bowl of a food processor. Add the sugar and pulse until the zest is finely chopped.
Beat the butter and lemon-sugar mixture in the bowl of an electric mixer on medium speed for about 2 minutes, until light and fluffy. Add the eggs, one at a time, blending well after each addition. Add the lemon juice and salt and blend until smooth.
Pour the mixture into a medium saucepan and place over low heat. Cook for about 10 minutes, stirring constantly, until a candy thermometer registers 175°F. Remove from the heat and transfer to a clean bowl. Cover with plastic wrap, pressing the wrap directly onto the surface of the custard. Chill in the refrigerator for at least 2 hours.
Combine the flour, sugar, baking powder, salt, and butter in the bowl of an electric mixer and mix on low speed for about 3 minutes, until the butter is the size of peas. Add the white chocolate. Gradually add the cream and mix just until the dough comes together.
Line two baking sheets with parchment paper. Turn the dough out onto a lightly floured work surface and roll into a 7-inch circle, about ¾-inch thick. Using a sharp knife, cut the dough into 12 wedges and place on the prepared baking sheets, spacing them 1-inch apart. Freeze for 15 minutes.
Meanwhile, preheat the oven to 375°F. Brush the tops of the scones with the egg wash. Bake for about 20 minutes, until lightly golden on top. Remove from the oven and let cool for 5 minutes. Transfer the scones to a wire rack and let cool completely.