The panna cotta can be made up to 1 day ahead; the longer it sets, the firmer it will become.
2 Tbsp. water, room temperature
1 envelope (1/4 oz.) unflavored powdered gelatin
1 vanilla bean, split lengthwise
1 1/2 cups half-and-half
1/3 cup clover honey, plus extra for serving
Pinch salt
2 cups plain low-fat Greek yogurt
2 grapefruits, peeled and segmented
6 butter cookies, coarsely crumbled
3 Tbsp. coarsely chopped pistachios