How to Make It
Preheat the oven to 425°. Sift the flour, baking powder, salt and baking soda into a large bowl. Using a pastry blender or 2 knives, cut the butter into the dry ingredients until the mixture resembles coarse meal.
In a small bowl, rub 1 tablespoon of the sugar with the grapefruit zest until the sugar is deep pink color. Add the grapefruit juice to the sugar and stir until the sugar is dissolved. Pour the grapefruit juice into the mixture in the large bowl and stir just until the biscuit dough comes together.
Scrape the biscuit dough out onto a lightly floured work surface and knead 2 or 3 times until smooth. Roll out the dough to a 1/2-inch-thick square. Cut the dough into 2-inch squares and transfer to a large baking sheet. Lightly brush the tops of the biscuits with milk and sprinkle with sugar. Bake the grapefruit biscuits for 15 minutes, or until they have risen and are browned on the bottom. Let the biscuits cool slightly before serving.
Make Ahead: The biscuits can be baked early in the day and stored at room temperature. Reheat in a 350° oven before serving.