Step 1

Bring cream, sugar, and grapefruit zest to a simmer in a large saucepan over medium-high, whisking occasionally, and cook (still whisking from time to time) until sugar is dissolved and cream is reduced by about a quarter (mixture should be just shy of 2 cups), 8–10 minutes.

Step 2

Remove pan from heat; immediately add grapefruit juice, lemon juice, and salt; whisk vigorously just until combined. Strain mixture through a fine-mesh sieve into a large measuring glass or a small bowl or other container with a spout.

Step 3

Pour into 4 glasses, dividing evenly (about ½ cup each) and chill until set, at least 3 hours.Do ahead: Possets can be made 2 days ahead. Cover and keep chilled.

Step 4

Cook guava paste and ¼ cup water in small saucepan over medium, whisking vigorously, until paste is melted and mixture is smooth, about 4 minutes. Remove from heat and add lemon juice, whisking well.

Step 5

Spoon jelly over set possets (about 1½ Tbsp. per portion) and smooth surface with a spoon. Chill until jelly is cool, 10–15 minutes.

Step 6

Just before serving, sprinkle tops of possets with salt.Do ahead: Possets with jelly (but without sea salt) can be made 1 day ahead. Cover and keep chilled.