01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
03 - In a large mixing bowl, beat the softened butter and sugar with an electric mixer on medium speed until light and fluffy, about 2 to 3 minutes.
04 - Beat in the egg, then the egg yolk, followed by the lemon zest, lemon juice, and vanilla extract, mixing until well combined.
05 - Gradually add the dry ingredients to the wet mixture, mixing on low speed just until incorporated. Do not overmix.
06 - Scoop tablespoon-sized portions of dough and roll into balls. Optionally, roll each ball in granulated sugar for a sweet coating.
07 - Place the cookie dough balls 2 inches apart on the prepared baking sheets.
08 - Bake for 9 to 11 minutes, or until the edges are set but the centers remain soft and slightly underbaked.
09 - Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.