Cut the lemons lengthwise and using a spoon, gently hollow out the halves keeping the lemon rinds intact. Transfer your lemon pulp to a sieve and press the juice into a bowl.
Put the sugar, blackberries, and lemon juice into a pot. Once boiling, reduce heat and let simmer for 5 minutes.
Remove the pot from the stove and using the back of a spoon, press the mixture through a sieve and into a bowl.
Once the mixture is lukewarm, stir in the butter. Once cooled, take a small amount from the bowl and with the back of a spoon, lightly coat the lemon rind bowls with the blackberry mixture.
Fold the whipped cream into the blackberry mixture.
Gently spoon the mixture into the lemon bowls and let freeze for 12 hours.