Chill the ice cream bowl for at least 24 hours or until solid. You should not hear any water sloshing around when you shake the bowl.

In a medium saucepan over medium heat, add the butter and brown sugar. Cook until the butter has melted and the sugar is dissolved and starting to bubble up, about 3-4 minutes. 

Whisk in ½ cup of the heavy cream and cook another 1-2 minutes until smooth. Remove the butterscotch from the heat and set aside. 

In a medium bowl, whisk together the egg yolks and granulated sugar until smooth and pale yellow in color. 

Add the milk and salt to a saucepan over medium heat. Bring the milk to a simmer and then remove from the heat. Be sure not to boil. Add about 1/2 cup of the hot milk to the egg and sugar mixture and whisk constantly until combined. 

Then, add the tempered egg mixture back into the saucepan with the remaining hot milk. Heat the saucepan on low. Stirring constantly, heat until the base has thickened enough to coat the back of a spoon. This should take a few minutes. 

Strain the mixture into a bowl to remove any pieces of egg that may have cooked. Add the remaining 1 cup of heavy cream, vanilla extract, and butterscotch, and stir to combine. Cover the bowl with plastic wrap making sure to have the plastic wrap touching the surface of the ice cream base. This will help keep it from forming a skin while chilling. Refrigerate for at least 3 hours, or until cold. 

Churn the ice cream mixture in the bowl of your ice cream machine according to your manufacturer’s instructions (mine took about 25 minutes), until it is thick and the consistency of soft-serve ice cream.

Spoon the ice cream into a loaf pan. Press plastic wrap or wax paper to the surface of the ice cream. Freeze for at least 4 hours, or until solid. Let sit at room temperature for about 5 minutes before serving.