Whisk unsweetened cocoa powder with 1/4 cup of the granulated sugar, then combine the cocoa mixture with heavy cream and whole milk in a saucepan. Heat over medium, whisking continuously, until steaming, about 5 minutes (visual cue: steam rises consistently across the surface).

Add finely chopped dark chocolate to the saucepan and whisk until fully melted and smooth. Continue whisking until no dark bits remain, about 2 minutes (visual cue: mixture looks glossy and uniform).

Whisk egg yolks with the remaining 1/2 cup granulated sugar until pale. Slowly whisk the hot chocolate cream into the yolks in a thin stream, 30–60 seconds (visual cue: mixture thickens slightly and turns lighter).

Return the custard mixture to the saucepan and cook over medium-low heat, stirring constantly, until it coats the back of a spoon at 175°F. Maintain gentle heat and stir for about 8 minutes (visual cue: a finger swipe leaves a clear path that doesn’t immediately flood back).

Strain the custard through a fine-mesh sieve, then stir in vanilla extract and salt. Stir until smooth, 1–2 minutes (visual cue: no grainy bits remain).

Cool completely over an ice bath, stirring occasionally as the temperature drops. Cool for about 20 minutes (visual cue: custard becomes cool to the touch and thickens slightly).

Refrigerate the custard at least 4 hours or overnight, covered. Chill until very cold throughout, 4 hours minimum (visual cue: custard is firm and chilled).

Churn the chilled custard in an ice cream maker until it reaches a soft-serve consistency. Churn about 20–25 minutes (visual cue: texture looks airy and holds peaks).

Transfer churned ice cream to a container and freeze until firm. Freeze at least 4 hours (visual cue: scoops hold shape and are thick enough to cut).