Preheat oven to 350F. Line two baking sheets with parchment paper.
In a medium bowl, whisk together the gluten-free flour blend, baking powder, and kosher salt.
In a large bowl, cream together the butter and granulated sugar for 1-2 minutes until light and fluffy.
Add the egg, lemon juice, lemon zest, and vanilla extract. Mix until combined.
Add the dry ingredients and mix until no more streaks of flour remain.
Scoop into 18 equal portions (about 1 oz. each), and roll into balls.
In a small bowl, combine granulated sugar and lemon zest. Roll the dough balls into the sugar mixture and set on the prepared baking sheets, leaving about 2 inches between each ball.
Bake at 350F for 7-8 minutes, until edges are set and the top of the cookies are no longer shiny. Do not overbake.