Combine 1 cup of heavy cream with cocoa powder in a medium saucepan, whisking until smooth. Heat gently over medium-low until boiling. Simmer for about 30 seconds, then remove from heat.

Add chopped dark chocolate to the cocoa mixture and stir until melted. Pour in remaining cup of heavy cream and whisk until uniform. Strain into a bowl to remove solids.

In a separate saucepan, combine milk, sugar, and sea salt. Warm until sugar dissolves, being careful not to boil.

Whisk egg yolks in a small bowl then gradually pour in about ½ cup warm milk while whisking. Return this mixture to the saucepan with the remaining milk.

Cook the custard, stirring constantly, until it thickens and coats the back of a spoon (about 170°F).

Strain the custard into the chocolate mixture. Stir in vanilla extract. Cover and refrigerate for at least 6 hours or overnight.

Pour the chilled mixture into an ice cream maker and churn according to instructions for 30 to 40 minutes.

Transfer churned ice cream into a container, smooth the top, and freeze for at least 6 hours. Press parchment paper onto the surface to prevent ice crystals.