First add the butter, ½ cup sugar, and grapefruit zest to a bowl and cream together until light and fluffy, about 1 minute. Then, add in the grapefruit juice, vanilla, and ground flax and mix together.
Next, add the sifted flour, baking powder, and salt to the mixing bowl. Mix until a smooth and thick cookie batter forms. Do not overmix or else you will form too much gluten and the cookies will not spread as much and be tough.
Add the bowl of grapefruit cookie dough to the fridge, cover, and let chill for 30 minutes- 1 hour.
Cover a baking sheet in parchment paper and preheat the oven to 350F. Next, remove the dough from the fridge and use a 2 Tbsp cookie scoop to form your cookie dough (40 grams per cookie of dough).
Roll into balls between your palms and then roll in the 2 Tbsp of sugar that was set aside so that each dough ball is coated.
Place the grapefruit sugar cookie dough balls onto the baking sheet. If you want flatter cookies, you can lightly press the dough down with your palm. Make sure that each cookie is kept about 2 inches apart since they will spread slightly while baking.
Bake for 11-13 minutes until bottoms are lightly browned and the cookie is cooked through but has slightly soft centers. The cookies will continue baking after they are removed from the oven so the centers should still be soft, but not glossy or raw, when they are removed from the oven.
Remove the cookie tray from the oven and immediately tap the tray on the counter to produce a more “wrinkly” cookie. Then, let the cookies cool on the baking sheet for 5-10 minutes before moving to a cooling rack to finish cooling.