In a separate bowl, beat the egg yolks and sugar until light and fluffy (1-2 minutes). (You can do this with a wire whisk or with a hand mixer.) Set aside while you heat the cream.

In a small saucepan gradually whisk the half and half cream into the sifted cocoa powder until you have a smooth paste. Place over medium-high heat and, stirring often, bring to the scalding point (the milk just begins to foam up). Remove from heat and gradually pour the scalding half-and-half mixture into the whipped egg yolk mixture, making sure you keep whisking constantly so the egg yolks don't curdle. If any lumps do form, strain the mixture. Then stir in the melted chocolate.

Place the mixture in a saucepan and place over medium-low heat. Stirring constantly, with a heatproof spatula or wooden spoon, cook until the custard thickens enough that it coats the back of a spoon (170 degrees F) (77 degrees C). (The term 'coat a spoon' is a technique used mainly as a way to test when an egg-based custard or sauce is done. A spoon, usually wooden, is placed in the custard and, when the spoon is raised, the film of custard on the back of the spoon will stay in place even when you draw a line with your finger through the middle of the custard.) 

Immediately remove the custard from the heat and continue to stir the custard for a few minutes so it does not overcook. At this point stir in the vanilla extract. Cover and let cool to room temperature and then refrigerate the custard until it is completely cold (several hours or preferably overnight). 

Transfer the cold custard to the container of your ice cream machine and process according to the manufacturer's instructions. Once made, transfer the ice cream to a chilled container and store in the freezer. If the ice cream becomes too hard, place in the refrigerator to soften for about 30 minutes before serving.

Chocolate Ice Cream Recipe:

2 ounces (55 grams) semisweet or bittersweet chocolate, chopped

4 large (80 grams) egg yolks, room temperature

1/2 cup (100 grams) granulated white sugar

2 cups (480 ml) half-and-half (a mixture of cream and whole milk that contains 10 - 12% butterfat)

1/3 cup (35 grams) unsweetened cocoa powder (regular or Dutch-processed)

1 1/2 teaspoons pure vanilla extract

Note: Instead of vanilla extract you can use 1/2 vanilla bean. Add the vanilla bean to the cream and cocoa powder mixture and bring to a boil.

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