In a medium heatproof bowl, whisk the yolks just to break them up, then whisk in half of the sugar (6 tablespoons). Set aside.

In a heavy stainless steel pan, combine the cocoa powder with the remaining sugar (6 tablespoons). Whisk in about 1/4 cup of the milk to form a paste, adding a little more milk as needed to make it smooth and uniform. Whisk in the remaining milk, cream, and salt, and put the pan over medium-high heat. When the mixture approaches a bare simmer, reduce the heat to medium.

Carefully scoop out about 1/2 cup of the hot cream mixture and, while whisking the eggs constantly, add it to the bowl with the egg yolks. Repeat, adding another 1/2 cup of the hot cream to the bowl with the yolks. Returning to the pan of cream on the stove, use a heatproof spatula to stir the cream as you slowly pour the egg and cream mixture from the bowl back into the pan.

Continue to cook the mixture carefully over medium heat, stirring constantly, until the mixture is thickened, coats the back of the spatula, and leaves a clear mark when you run your finger across it.

Strain the base through a fine-mesh strainer and into a clean container. Mix in the vanilla extract. Set the container into an ice bath, wash your spatula, and use it to stir the base occasionally until it is cool. Then cover with plastic wrap and refrigerate the base for at least 2 hours or overnight.

To freeze: Churn in your ice cream machine according to the manufacturer's instructions. While the ice cream is churning, put the container you'll use to store the ice cream into the freezer. Enjoy right away or, for firmer ice cream, freeze for at least four hours.