How to Make It
Preheat the oven to 300°F. Arrange the already-baked cookies on a baking sheet and bake until fragrant, 6 to 7 minutes; cool completely.
In a small (2-quart) saucepan, bring the grapefruit juice to a boil, then reduce the heat to medium-high and cook until the juice has reduced to ½ cup, pouring the liquid into a measuring cup every 5 minutes to check the amount, for 15 to 20 minutes. Remove from the heat and cool to room temperature in the saucepan, 30 minutes.