01 - Sprinkle gelatin over grapefruit juice in a small bowl. Let stand for 5 minutes until gelatin absorbs liquid and softens.
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 until completely dissolved.
04 - Pour mixture through a fine mesh sieve into a clean bowl to remove grapefruit zest, ensuring smooth texture.
05 - Divide panna cotta mixture evenly among 6 ramekins or serving glasses.
06 - Let cool to room temperature, then cover and refrigerate for at least 4 hours until firm.
07 - If desired, toss grapefruit segments with 2 tablespoons sugar and let stand for 10 minutes to macerate.
08 - Top each set panna cotta with grapefruit segments and fresh mint leaves just before serving.