The saucepan should have at least a one-liter capacity, leaving enough room for stirring.
Once the butter starts to get frothy, add the brown sugar and the heavy cream, then stir til completely incorporated. Turn the heat down to medium or low-medium and allow the butterscotch to reduce for 2 minutes.
375 grams dark brown sugar, 355 milliliters heavy cream
Add 1 teaspoon of the sea salt to the butterscotch and stir to incorporate.
1½ teaspoon flaked sea salt
Reserve 1 cup of the butterscotch in an airtight container. Once this has cooled add the remaining ½ teaspoon of sea salt & set aside to use later in layering the ice cream
1½ teaspoon flaked sea salt
Make the Custard Base
Add the whole milk to the butterscotch in the saucepan and stir until completely incorporated.
475 milliliters whole milk
Strain the milk mixture, saving the strained pecans to reincorporate later (or toss them if you prefer not to have pecan chunks).
While the butterscotch/milk mixture is still hot, temper the egg yolks by slowly pouring in a third of the butterscotch/milk mixture while simultaneously whisking the egg yolks. Whisk until completely incorporated.
Add the rest of the milk mixture slowly to the tempered mixture. Be sure to scrape the sides and bottom of the bowl with the whisk to make sure all the egg yolks are incorporated.
Give the saucepan a quick rinse and dry off excess water, then return the tempered custard mixture to the saucepan. Stir over medium to medium-high heat just until the hot mixture coats the back of a spoon (or until it reaches a temperature of 180°F)
Do NOT let the custard boil or bubble, and don't crank the heat up to high to make it go faster. You run the risk of curdling the custard.
Transfer the custard to a heat-resistant bowl and reincorporate the strained pecans from earlier. Cover with plastic wrap or a lid and allow to chill in the refrigerator for at least 6 hours.
After the custard base has been chilled, give it a stir to distribute the pecan chunks, then pour it into your ice cream machine and churn according to the manufacturer's instructions (about 35 minutes in my Whynter ice cream maker).
Assembling the Ice Cream
In a freezer-safe & airtight container, layer some of the ice cream and then drizzle or dollop some of the reserved salted pecan butterscotch from earlier.
Repeat these layers until all of the ice cream and butterscotch have been used, then use a skewer or a knife to further distribute the butterscotch throughout all the ice cream!
Allow the finished ice cream to set in the freezer for at least 2 hours before serving.
Don't chop the pecan pieces too finely. Tiny flecks can make the ice cream seem gritty instead of having nutty chunks. To avoid this, pass the pecans through a fine-meshed sieve to get rid of tiny pieces.
Churn the ice cream in the coolest spot in your kitchen (or home). This is especially important if using a canister-style ice cream maker, so that a warm environment doesn't work against your ice cream churning. You can also use an appliance cover or a clean towel as an extra layer of insulation.
You can also layer the ice cream in a bread/loaf pan lined with parchment paper and then seal the top with plastic wrap. However, I only recommend this if you will be serving all of the ice cream within 3-4 days, since it will be more prone to freezer burn and absorbing aromas from other foods in your freezer.