If using gel-canister ice-cream maker, freeze canister overnight.
Place a large bowl and strainer on heatproof surface near stove. Combine milk and yolks in a heavy, nonstick, medium-size saucepan. Stir in sugar and cocoa. Cook over medium-low heat, stirring constantly with rubber scraper, until instant-read thermometer just reaches 190 degree F, about 15 to 20 minutes (do not let boil), or until slightly thickened. Strain immediately into bowl; stir in salt.
Whisk in chocolate until incorporated. Stir in vanilla. Place wax paper onto custard; refrigerate until chilled, about 4 hours.
To freeze, add cream to custard, stirring to combine. Pour in ice-cream maker; process 30 minutes as per manufacturers directions.
Scrape into a 1-1/2-quart container; freeze to firm completely, about 2 hours.
Makes 10 servings.