Whisk together your flour, salt, and citric acid in a bowl and set it aside.
Cream your softened butter and sugar together in a stand mixer with the whisk attachment for 1-2 minutes until it's light and fluffy.
Add in the room temperature egg, lemon extract, and zest and mix until combined.
While mixing on low, add the flour mixture to the egg mixture until the dough just begins to stick together. Do not over-mix.
Divide the dough in half and roll into logs about 3" in diameter. Roll in sanding sugar sprinkles if you desire. It adds a nice sparkle to the finished cookie.
Wrap the dough in plastic wrap, and refrigerate for two hours or overnight.
Preheat your oven to 350ºF and line a baking sheet with parchment paper.
Slice your lemon shortbread dough into 1/4" thick slices. Arrange them on a baking sheet lined with parchment paper with about 1" of space in between each cookie. They won't spread.
Bake at 350°F (180°C) for 12-14 min until slightly golden brown around edges.
Let your cookies cool for 5 minutes then carefully transfer them to a cooling rack to fully cool down.