In medium saucepan over medium-low heat, whisk together chopped chocolate bar, unsweetened cocoa, sugar, and milk. Whisk constantly until mixture starts to simmer and bubble around the edges of the pot. Remove from heat.
In small bowl, whisk the egg yolks then quickly pour in about 1/2 cup of the chocolate mixture and whisk until completely combined. Pour the tempered egg mixture back into the saucepan with the chocolate mixture and whisk briskly to combine.
Return saucepan to medium-low heat and continue cooking and whisking until mixture returns to a simmer. Remove from heat and allow to cool.
Stir in heavy cream, salt, and vanilla. Cover and refrigerate 8 hours or overnight.
Follow instructions on your ice cream maker to freeze ice cream mixture. Depending on capacity of your ice cream freezer, you may need to divide mixture into batches for freezing.
Transfer ice cream to storage container and place in freezer until firm, if desired.