Preheat oven to 350.
Line a baking sheet with parchment paper and set aside.
Using a stand mixer (preferred) or a hand mixer, beat the sugar and butter for about one minute, until creamed together well.
Next, add the lemon juice, lemon zest, egg, and vanilla extract, and beat for about another minute.
Add the flour and baking powder, and beat again, but do not over mix—you just want the ingredients to be incorporated.
Refrigerate the dough for at least 30 minutes, so that the cookies don’t spread too much.
Remove dough from the refrigerator, form 12-14 balls and place them on the baking sheet.
Sprinkle with extra sugar on top.
Bake 10-11 minutes—they should be very soft, but they will firm up as they cool.
Sprinkle extra lemon zest and sea salt flakes on top.