Step 1

Whisk together milk, cream, sugar, cocoa, and salt in a medium saucepan. Cook over medium-high, whisking constantly, until sugar and cocoa dissolve, about 3 minutes. Add nori, and cook until the sheets are softened, another 7 to 8 minutes. Cover and remove from heat. Let stand 20 minutes. Pour milk mixture through a fine wire-mesh strainer into a bowl, discarding solids; remove and reserve 1/2 cup of mixture. Return remaining milk mixture to saucepan.

Step 2

Place bittersweet chocolate in a large bowl. Whisk the egg yolks in a small bowl. Gradually stir 1/2 cup reserved milk mixture into egg yolks; return the saucepan to medium-high. Slowly whisk egg yolk mixture into milk mixture. Cook, stirring often, until mixture thickens slightly and coats the back of a spoon, 12 to 14 minutes. (Do not boil.) Immediately remove from heat, and pour over bittersweet chocolate; whisk well to combine. Cool to room temperature. Partially cover, and chill 3 hours.

Step 3

Pour mixture into freezer bowl of a 1 1/2-quart electric ice-cream maker, and proceed according to manufacturer's instructions. (Instructions and times will vary.) Transfer to a freezer-safe container, and freeze until ready to serve. (Ice cream is best when eaten within 24 hours.) Garnish with coarse sea salt, if desired.