Melt the butter in a medium saucepan over medium heat until bubbling and golden. Add dark brown sugar, whisking for 2 to 3 minutes until smooth and bubbly.

Stir in heavy cream, bringing the mixture to a gentle boil. Simmer for 3 to 4 minutes, keeping an eye to prevent burning.

Remove from heat and whisk in heavy cream and whole milk. Heat until the mixture reaches 165°F.

In a separate bowl, whisk egg yolks and granulated sugar until combined and frothy.

Gradually pour in 1 cup of the hot butterscotch mixture into the egg yolk-sugar blend, whisking continuously.

Return the tempered mixture back into the saucepan and whisk together.

Cook the mixture again on medium heat until it reaches 165°F, thick enough to coat the back of a spoon.

Transfer the mixture to a bowl, whisk in vanilla extract and kosher salt. Allow it to cool before chilling in the fridge.

Churn the chilled mixture in an ice cream maker according to instructions for 25 to 30 minutes.

Pack the churned ice cream into a freezer-safe container and freeze for at least 2 hours.

Serve the butterscotch ice cream in bowls or cones, adding toppings if desired.