I think the best way to crush the coriander seeds is to place them in a heavy-duty bag and crack them with a heavy object, like a rolling pin. You could also use a mortar and pestle, but don’t crush too much.

2 C. whole milk
1 ¼ C. heavy cream
2/3 C. cane sugar
1/8 C. light corn syrup
1 1/2 T. crushed coriander seeds
1 T. plus 1 tsp. cornstarch
1 ½ oz. cream cheese, softened
1/8 tsp. kosher salt
1/4 to 1/2 C. blackberry sauce (see below)

Warm 1 C. milk, sugar, corn syrup, cream, and coriander seeds in a medium saucepan. Once warm (do not boil) remove from heat, cover and let steep at room temperature for one hour. When steeping is almost complete, fill a large bowl with ice water. In a small bowl, mix 2 T. the milk with the cornstarch. In a medium bowl, whisk the cream cheese and salt until smooth. After steeping is complete, add remaining milk and begin rewarming over medium heat. Bring milk mixture to a boil. Cook over moderate heat for about 4 minutes to remove some moisture. Remove from heat and off heat gradually whisk in the cornstarch mixture. Return to a boil and cook over moderately high heat until the mixture is slightly thickened (draw a line on a spoon), about 1 minute. Gradually pour the hot milk mixture through a sieve into the cream cheese and whisk until smooth. Pour into a gallon bag, seal tightly, and set baggie in the ice water bath, cover with some ice, and let stand until cold, about 20 minutes or longer. Once ice cream base is chilled, pour ice cream base into an ice cream maker and freeze according to the manufacturer’s instructions. Spoon a layer of blackberry sauce (see below) into a container and lightly spoon a layer of ice cream on top. Continue to alternate layers of sauce and ice cream until the container is full; drizzle a little more sauce on top. Do not swirl with a spoon. Press a sheet of plastic wrap directly onto the surface of the ice cream (to prevent ice crystals) and close with an airtight lid. Freeze until firm, at least 4 hours.

Blackberry sauce

Sauce can be prepared in advance and frozen until ready to use.

2 C. blackberries, fresh or frozen
1 C. sugar

Place both ingredients in a small saucepan over medium heat. Stir occasionally. Berries will soon begin to release their juice, keep stirring to combine juice and sugar. Simmer until sugar is dissolved and most juice has been released from berries, about 8 minutes. Cool completely before adding to ice cream.