Portion out all of your ingredients so you're ready to go. Crack 42 g egg yolks into a small bowl and whisk well. Set aside.
Prepare an ice bath by filling a large bowl with cold water and several handfuls of ice cubes. Set aside.
Set a fine-mesh sieve over a smaller bowl you set aside for the ice bath.
Grab a saucepan that holds at least 2 quarts of liquid. Pour in 150 g cane sugar, 60 g skim milk powder and 0.3 g guar gum (if using). Whisk to combine.
Pour 400 g whole milk into the pan with the sugar/SMP mixture, and whisk until the sugar/SMP have mostly dissolved.
Add 400 g heavy cream and whisk again until the mixture feels smooth.
Gently heat the pot over medium-low, whisking constantly, until it reaches 110°F on an instant-read thermometer. Once it reaches temperature, immediately remove it from the heat.
Take a ladleful of the warm cream mixture, and stream it slowly into the egg yolks, whisking vigorously to prevent curdling.
Once the mixture is smooth, pour the egg/cream mixture back into the saucepan, and whisk well until the liquid takes on a yellowish hue.
Return the pan to medium-low heat, and continue warming, whisking constantly, until it reaches 165°F. Once at temperature, immediately remove it from the heat.
Strain the hot custard into the bowl, then stir in 1 g kosher salt.
Place the bowl into the ice bath and whisk often for about 15 minutes, or until the custard reaches about 75°F.
Cover the bowl with plastic wrap and place it in the refrigerator for 4-6 hours, or overnight, to age and chill.
Once the custard has chilled, pre-chill your ice cream maker according to the instructions. Pour in the custard with the machine running. Allow the custard to churn according to the machine's instructions. This will take anywhere from 20-45 minutes, depending on the machine.
Scoop the ice cream into pints with wax paper pressed onto the surface to prevent ice crystals, and set in the freezer for at least 4 hours, or overnight, to fully set and age.