Prepare a 1 quart souffle dish by creating a collar to go around the rim to support the souffle while it freezes; cut a sheet of waxed paper long enough to go around the dish's rim, then fold in thirds, lengthwise (about a 4 inch wide ribbon).
It should extend about 3 inches above the rim; secure with a rubber band or tape (or you could prepare 8 ramekins with the collar 1 1/2 inches above each one).
In a large saucepan, over medium heat, cook the blackberries and 1/3 cup sugar until they soften and burst, about 10 minutes; puree in a food processor, then strain the seeds out (you should have about 2 cups of puree).
Stir in the liquer and chill.
In a small saucepan, heat remaining 2/3 cup sugar and water, over medium heat, to the soft-ball stage, 235 - 240 degrees, about 15 minutes.
Stir until the sugar dissolves, then stop stirring, but use a small brush, moistened with water, to wash down any crystal that may form on the sides of the pan.
Meanwhile, beat the egg whites in a stand mixer, using the whisk attachment, until stiff peaks form; while running pour the sugar mixture, in a slow steady stream, into the egg whites taking care not to hit the whisk attachment and splatter.
The egg whites will swell, then subside; continue beating until the egg whites cool to room temperature, about 8 minutes.
In a large bowl, beat the whipping cream, until soft peaks form, using a whisk or hand mixer.
Stir 1/3 of the egg white mixture into the blackberry puree, then pour the blackberry mixture into the remaining egg whites and gently fold until well combined.
Gently fold the whipping cream into the egg whites, then spoon the mixture into the prepared souffle dish or the ramekins; it should come about 1/2 up the waxed paper collar.
Smooth the top and freeze until very firm, 4 hours to overnight; if frozen overnight, it will need and hour in the refrigerator before serving.
If freezing for more than a day, press plastic wrap onto the surface once it is frozen to store.
To serve, remove the collar and cut into thin slices.