Step 1

Add butter, brown sugar, and cream to a small saucepan and heat over medium heat.

Step 2

Bring to a low boil and continue cooking, stirring occasionally, for about 5 minutes.

Step 3

Remove from heat and stir in vanilla and salt. Set aside to cool -- sauce will thicken as it cools.

Step 4

To make the milkshakes, add milk, ice cream, and ½ cup of cooled butterscotch sauce to a blender. Blend just until smooth and immediately pour into two glasses to serve. If desired, top with whipped cream, more butterscotch sauce, and more sea salt.