In a medium sized bowl, whisk the egg yolks with 1/4 cup of the sugar until the color of the egg yolks starts to lighten. Set aside until needed. 4 large egg yolks, 3/4 cup granulated sugar
Add 1 cup of the heavy cream, the milk, remaining sugar, cocoa powder to a large sauce pan. Stir them together. 2 cups heavy cream, 1 cup whole milk, 3/4 cup granulated sugar, 1/4 cup cocoa powder
Heat the mixture over medium until the mixture just starts to bubble around the edges.
Slowly add 1/2 cup of the chocolate cream mixture into the egg yolks, while you stir them together. Add another 1/2 cup of the mixture, while stirring the whole time.
Slowly pour the chocolate custard mixture back into the sauce pan and continue to heat it over medium heat.
Add in the chocolate pieces, and let them melt into the mixture, stirring until smooth. 4 oz semi sweet chocolate
Cook the mixture for another 5 minutes or so, until the mixture starts to thicken, enough to coat the back of a spoon.
Remove the mixture from the heat and stir in the vanilla extract, salt, and remaining cup of cream. 1/2 tsp vanilla, 1/4 tsp salt
Pour the mixture through a strainer into a large bowl, on top of an ice bath. Let it sit for about 20 minutes, stirring occasionally as the mixture starts to cool down to room temperature.
Cover the bowl with plastic wrap, or a lid and place it in the fridge to chill for at least 2 hours, up to overnight.