¼cup(2fluid ounces)whole milk, at room temperaturedivided
2 ½ouncesbittersweet or semisweet chocolatechopped
Scant ½teaspoonkosher salt
Prepare your ice cream machine
If necessary, freeze the bowl of your ice cream machine for the recommend time.
Prepare the mixture
In a small, heavy-bottom saucepan, place the granulated sugar and 1/4 cup of the whole milk. Over medium-low heat, whisking frequently, bring the mixture to a very gentle simmer.
Continue to simmer the mixture until sugar is dissolved, and the mixture has become syrupy.
Shake the can of evaporated milk vigorously, then open the can and pour it into a 4 cup or larger measuring cup or mixing bowl with a pour spout. Add the remaining whole milk and the sugar mixture, and the optional vanilla, and whisk to combine.
Add the optional guar gum, and whisk until it's fully dissolved into the liquid.
Pour into container with remaining milk, evaporated milk and vanilla, and whisk to combine well. Place the measuring cup or bowl into the refrigerator and chill until cold (about an hour).
Pour the chilled mixture into the bowl of your ice cream maker, and process according to the manufacturer's directions, or until thickened.
Transfer the processed ice cream to a freezer-safe container, cover tightly, and freeze for about 4 hours or until it reaches the consistency you like best.
Make the hot fudge sauce
In a medium-size, heavy-bottom saucepan, place the granulated sugar, cream of tartar, cocoa powder, and whole milk, and mix to combine well.
Place the saucepan over medium heat, and bring to a simmer. Reduce the heat to medium-low, and continue cooking until the mixture thickens and reduces by almost one-quarter (about 3 minutes). It will first seem thinner as the sugar melts into the liquid, then eventually thicken.
Remove the pan from the heat, add the chocolate, butter and salt, and whisk to combine well. Transfer to a heat-safe container with a lid, and allow to cool slightly before serving.
Store in the refrigerator, and warm in a water bath on the stovetop or in the microwave before serving.