Preheat oven to 400 degrees.

In a food processor or blender, mix zest and granulated sugar together until it's well-combined.

Roll your cookie dough into 1-inch balls. Dip the bottom half of them into the zest/sugar mixture.

Arrange them on the cookie sheet, sugar-side down, spaced 3 inches apart. Dip a drinking glass in some flour and press the cookie balls flat until they are about 1/4 inch thick discs.

Bake for 8-12 minutes (use the suggested time on your cookie recipe), until they are just slightly brown on the edges (try not to overbake them). Let them cool completely.

While they cool, whisk together the powdered sugar and 2 Tablespoons of grapefruit juice. If it's too thick to drizzle, add the last Tablespoon, bit by bit. Whisk in one drop of food coloring if desired.

Drizzle the glaze over the cooled cookies and let set briefly before serving.