Cream together butter, sugar, brown sugar, lemon juice, lemon zest, salt, and vanilla with a mixer until smooth.
Add the flour, and stir just until incorporated.
Scrape the dough onto a sheet of parchment paper, and use your hands to form the dough into a log that is 1 ½ inches in diameter.
Wrap the log in the parchment paper, then twist both sides to close tightly.
Chill in the refrigerator for at least 3 hours, and preferably 24 hours.
Preheat oven to 350 degrees.
Spread sparkling sugar on a plate.
Unwrap cookie dough log, and cut into two shorter logs so they are easier to work with.
Paint the outsides with egg white.
Roll logs in sparkling sugar until completely covered.
Cut into slices slightly thinner than ½-inch.
Place shortbread slices on a parchment paper-lined baking sheet, with an inch or two of space between cookies.
Bake for 12-13 minutes, until just golden brown on the edges.
Let cool for 1-2 minutes on the baking sheet on a cooling rack, before moving the cookies themselves onto the rack. Let cool completely before storing.
Video
Notes
Note on Video: FYI, I mention the incorrect amount of flour in the video in this recipe card! Go with what the printed recipe says, and always use the weighted measurements if possible. Make-Ahead Tips: These cookies taste the best if you make the dough a day before baking and then bake a day before serving! And, you can even make them further in advance. Make the dough up to 3 days before using, and keep in the refrigerator. Or, freeze the parchment logs for up to 3 months. Defrost in the refrigerator for 24 hours, then continue with the recipe.Expert Tips: If you want to bake all your cookies at one time, you’ll need two cookie sheets. Put one on the top oven rack and one on the bottom, then switch halfway through baking. Always good to rotate the sheets back to front while you’re in there too. Wondering the best knife to slice shortbread cookies? We prefer a large serrated knife, like a bread knife. The one we use is the Victorinox Swiss Army Bread Knife.