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1
In a 3-quart stainless steel saucier, combine blackberries, sugar, and salt. Using a metal spatula, crush the berries until the sugar dissolves (the spatula's comparatively sharp edge will minimize splashing compared to the dull edge of a potato masher). Using a kitchen scale, weigh the pot and fruit together, then make note of that number to track the reduction. Cook over medium heat until bubbling hot, then simmer, stirring constantly, until the mixture has reduced by 14 ounces (395g). The time required will vary depending on the size, shape, and type of cooking vessel, as well as the size and output of the burner, but expect about 30 minutes, and adjust heat as needed to proceed at a similar rate. (Try not to over-reduce the fruit, as it can produce unwanted flavors, but if you do accidentally do it, you can add back just enough water to correct the weight.)
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2
When reduced by 14 ounces, strain into a large bowl through a fine-mesh stainless steel strainer. Press and stir the blackberries with a flexible spatula to extract their juices, until there's nothing left in the sieve but about 10 ounces (1 heaping cup; 285g) seedy pulp, with 20 ounces (565g) blackberry puree in the bowl.
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3
Discard the blackberry pulp. Stir the cream, cinnamon, and alcohol (if using) into the concentrated blackberry puree, along with lemon juice (if needed) to brighten the flavor. Cover and refrigerate until no warmer than 40u00b0F, then churn in an ice cream maker according to the manufacturer's directions. Meanwhile, place a 1-quart container and flexible spatula in the freezer.
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4
When ice cream looks thick and light, shut off the machine and, using the chilled spatula, scrape ice cream into the chilled container. Enjoy as soft-serve, or cover with plastic pressed directly against surface of ice cream, then close lid and freeze until firm enough to scoop.
