Remove wide strips of lime zest from limes. Bring cream, lime strips, salt and sugar to a boil in a medium saucepan over medium-high heat.
Reduce heat and simmer, stirring constantly to prevent mixture from boiling over. Cook until mixture is slightly thickened, about 5 minutes. Remove from heat and discard lime strips.
Stir in 1/2 cup fresh lime juice and cool 10 mins.
Fill individual dessert cups. Cover and chill possets until set, at least 4 hours.
For the compote:
Drain blackberries and reserve juice. Set Blackberries aside along with 1/4 cup Blackberry juice. Combine remaining juice from blackberries and Crème de Cassis in a medium saucepan and cook on medium-high until juice has reached a low simmer.
Add cornstarch to reserved blackberry juice, and stir until completely dissolved.
Add cornstarch mixture to simmering juice, and stir well until liquid is thickened, about 5 minutes.
Add blackberries and lemon juice to thickened sauce and stir until berries are enrobed in sauce.
For the assembly:
Top with Blackberry Cassis Compote and whipped cream. Garnish with Lime Zest.