In a small pan over medium-low heat, combine your butter, brown sugar, vanilla extract and 1/4 cup heavy whipping cream. Stirring frequently, cook your butterscotch sauce for 15 minutes, or until it coats the back of a spoon. Set aside to cool to room temperature.

In the bowl of a stand mixer, whip the remaining heavy whipping cream until it holds stiff peaks. Add the sweetened condensed milk and continue whipping until thick and fluffy. Add your cooled butterscotch sauce (leaving a small amount for drizzling over the top) and whip until just combined.

Pour your butterscotch ice cream into a loaf pan. Drizzle the remaining butterscotch over the top and swirl with a butter knife. Place your ice cream in the freeze and allow to set overnight (or at least 5-6 hours).