01 - Sprinkle gelatin over grapefruit juice in a small bowl. Let stand for 5 minutes until gelatin absorbs liquid and softens completely.
02 - Combine cream, milk, sugar, vanilla extract, and grapefruit zest in a saucepan. Heat over medium heat, stirring constantly, until sugar dissolves and mixture is hot but not boiling.
03 - Remove saucepan from heat. Add bloomed gelatin mixture and whisk vigorously until completely dissolved and smooth.
04 - Pour mixture through a fine mesh sieve into a clean bowl to remove grapefruit zest, ensuring a silky smooth texture.
05 - Divide panna cotta mixture evenly among 6 ramekins or serving glasses, leaving space at the top for garnish.
06 - Cool to room temperature uncovered, then cover with plastic wrap and refrigerate for at least 4 hours or overnight until completely firm.
07 - Toss grapefruit segments with 2 tablespoons sugar in a small bowl. Let stand for 10 minutes to macerate and release natural juices.
08 - Top each set panna cotta with macerated grapefruit segments and fresh mint leaves just before serving.