Whisk together 1 cup of heavy cream with cocoa powder in a medium saucepan over medium heat until it boils, then simmer for 30 seconds.

Stir in chopped chocolate until melted and smooth, then add remaining heavy cream and strain through a fine mesh strainer.

Warm 1 cup of milk, sugar, and sea salt in another saucepan until sugar dissolves without boiling.

Whisk 4 egg yolks in a small bowl and temper by adding ½ cup of warm milk mixture gradually.

Pour tempered yolks back into saucepan, cook on medium heat until thickens to 170°F.

Strain custard into the chocolate base, add vanilla extract, and stir until combined.

Cover and refrigerate chocolate mixture for at least 6 hours.

Churn in an ice cream maker for 30-40 minutes until thick and creamy.

Transfer to a container, smooth top, and freeze for at least 6 hours before serving.