Combine the heavy cream and milk in a saucepan over medium heat until it begins to steam, not boil.

In a bowl, whisk together the sugar and egg yolks until pale and fluffy.

Gradually temper the yolk mixture by adding a bit of the hot milk mixture, whisking constantly.

Return the entire mixture to the saucepan and cook over low heat, stirring constantly, until it thickens enough to coat the back of a spoon.

Remove from heat and stir in the finely chopped dark chocolate and cocoa powder until smooth.

Pour the mixture through a sieve into a bowl, cover, and chill in the refrigerator for at least 3 hours.

While the mixture chills, prepare the hazelnut brittle. Combine granulated sugar and water in a saucepan. Heat without stirring until the sugar dissolves and reaches a golden caramel color.

Stir in the hazelnuts and then quickly pour the mixture onto a baking sheet lined with parchment paper, spreading it thin. Allow to cool and harden.

Break the hardened brittle into small pieces.

Churn the chilled ice cream mixture in an ice cream maker according to the manufacturer's instructions.

Once churned, fold in the broken hazelnut brittle pieces.

Transfer the ice cream to a freezer-safe container and freeze until firm, about 2 hours.

Serve the chocolate ice cream garnished with additional hazelnut brittle pieces and a sprinkle of sea salt.