In a medium sized, heavy bottomed pan, bring cream and sugar to boil over a medium-high heat, stirring until sugar dissolves.
Reduce heat to medium, and boil 3 minutes, stirring constantly, adjusting heat as needed to prevent mixture from boiling over. Remove from heat.
While posset is cooling, fill each serving glass with 1 Tablespoon graham cracker crumbs
Stir in lime juice and half the lime zest, and allow to cool for 10 minutes.
Pour posset into serving glasses or ramekins and cover with plastic wrap.
Chill for one hour or more. Posset can be made a day before.
To garnish lime posset use remaining zest, more graham cracker crumbs, small lime wedges or fresh berries.