1. Warm the milk chocolate and the heavy cream in a large heatproof bowl set over a pan of simmering water. Stir until the chocolate is melted, then remove from heat and set aside with a fine-meshed strainer nearby.
2. In a saucepan, warm the milk with the sugar and salt. Stir together the egg yolks, then gradually add some of the warm milk to them.
3. Pour the warmed eggs back into the saucepan and cook over low heat, stirring and scraping the bottom constantly with a heatproof spatula, just until the custard thickens enough to lightly coat the spatula, 3 to 5 mins. Strain the custard into the milk chocolate mixture and stir together. Stir in the cognac. Chill thoroughly.
4. Place the mixture in your ice cream maker and freeze according to teh manufacturer's instructions. Stir in the cacao nibs when you remove the ice cream from the freezing chamber.