In a medium bowl, lightly whisk together the yolks and half of the sugar.
In a non-reactive saucepan combine the cream, 1 cup of the milk, the remaining sugar, and the salt.
Split the vanilla bean in half, lengthwise, to expose the tar-like seeds inside the pod. Scrape the seeds loose with a knife; add the bean and the seeds to the cream mixture.
Heat the cream over medium-high heat until just at a boil. Remove from the heat. Gradually pour the hot liquid into the yolks, while whisking constantly. Return the cream-egg mixture to the saucepan. Cook over medium-low heat, stirring the mixture constantly, with a wooden spoon in a figure-8 motion, until thickened, about 10 minutes. When the ice cream mixture is properly cooked it should coat the back of a spoon, be satiny thick, and be free of any bubbles on the surface. (If you taste the ice cream base it should have a slight egg-y taste.) When thickened, pour in the reserved milk to prevent the mixture from overcooking. Strain into a medium bowl. Add the cognac if desired.
Fill another bowl with ice. Set the bowl of ice cream base in the ice and stir until thoroughly chilled. Freeze the base in an ice cream machine according to the manufacturer's instructions. (Take care not to over churn the ice cream or it will get a grainy texture.) Transfer the ice cream to the freezer to set up for at least 1 hour. Serve.
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