Finely zest the lemons, then juice them to yield 0.33 cup lemon juice; strain out any seeds and set both zest and juice aside.
Combine the heavy cream and granulated sugar in a medium saucepan. Bring to a gentle boil over medium heat, stirring to dissolve the sugar, then boil 3 minutes, stirring often, until slightly thickened and the cream lightly coats the back of a spoon.
Remove from the heat. Whisk in the reserved lemon zest and lemon juice until smooth. Let stand 10 minutes to cool slightly and begin to thicken.
Strain the mixture through a fine-mesh sieve into a jug or measuring cup, pressing gently to extract as much as possible; discard the zest.
Divide evenly into 6 small ramekins or glasses. Let cool at room temperature 20 minutes, then refrigerate until set, 4–6 hours; the possets should jiggle slightly when tapped but not flow.