In a mixing bowl , preferably one with a spout (I use a Pampered Chef Batter Bowl), whisk the egg yolks until they lighten in color. Set them next to the stove.

Next, whisk the cocoa powder and 1 cup of the half and half together in a saucepan. Set the pan over medium heat and whisk until smooth, then whisk in remaining half and half and cream. Bring to a simmer, then remove from heat.

Add the sugar to the egg yolks and whisk until smooth, then whisk in about ⅓ cup of hot cream mixture to temper. Add another ⅓ cup, whisking until combined, then add another ⅓ cup and whisk. Gradually add all remaining cream mixture to egg mixture.

Pour mixture back into saucepan and cook over low to medium low heat, stirring frequently, until the mixture reaches 170 to 175 degrees F. It won't thicken very much at this point, but it should coat the back of the spoon. Pour the mixture into a clean bowl and allow to sit at room temperature for 30 minutes. Stir in the vanilla extract.

Cover mixture and refrigerate for at least 4 hours (I recommend 8).

Pour into an ice cream maker and process according to the manufacturer's directions. This should take approximately 25 minutes.

Transfer to a lidded tub and freeze until ready to serve.