Next, use a soup ladle or large spoon to transfer a small amount of the warm chocolate mixture to the small bowl with the egg yolks; add the hot liquid all at once to the yolks, whisking the yolks constantly.
Then transfer the entire contents of the small bowl back into the saucepan and return to medium-high heat. Heat the chocolate mixture, stirring constantly, until the liquid begins to thicken slightly. Remove the pan from the heat.
Strain the chocolate mixture through the fine mesh sieve and into the medium bowl that already contains the fat free milk. (The sieve catches any egg pieces that might have 'cooked,' so if you don't have a sieve or don't care about if you have some egg in your ice cream, just skip this and add the hot chocolate liquid directly to the milk.)
Finally, stir in the vanilla until everything is smooth, and then transfer the bowl to the fridge to let it cool completely (about 3 hours). This is the point when you can add in your optional mix-ins.