Obolsky includes egg yolks for a supremely silky texture, but they need to be slowly tempered into the hot milk and sugar mixture so they don't curdle. You then want to cook the mixture slowly over low heat, stirring constantly, until it's thick enough to coat the back of a spoon.
At this point, you can customize: Strain it through a fine mesh sieve for a classic vanilla, or transform it by whisking in melted bittersweet chocolate or ground coffee before straining.
If you take the coffee route, be sure to strain it right away—if the coffee steeps too long, it will become bitter.
Obolsky recommends allowing the final mixture to rest in the fridge for 24 hours to let the flavors mellow and the proteins relax, which makes for a creamier final product.
Once it's spun in the machine, you can fold in anything you can imagine, like chocolate chips, peanut brittle, or cookie crumbles.