Whisk together the brown sugar, cornstarch, and salt in a medium saucepan until well mixed.

Gradually pour in the whole milk, whisking constantly for an even mixture.

Set the saucepan over medium heat and stir continuously until the mixture thickens and bubbles gently, about 5-7 minutes.

In another bowl, lightly beat the egg yolks.

Slowly add about 1/2 cup of the hot milk mixture to the egg yolks, whisking continuously to temper them.

Pour the tempered yolk mixture back into the saucepan with the remaining milk mixture.

Cook for an additional 2 minutes over medium heat, stirring constantly until the mixture is very thick.

Remove from heat and stir in the unsalted butter and vanilla extract until everything is smooth.

Pour the butterscotch filling into your pre-baked pie crust. Allow it to cool at room temperature, then refrigerate for at least 4 hours or until set.

In a chilled bowl, whip the heavy cream with the powdered sugar until stiff peaks form.

Spread the whipped cream over the chilled butterscotch pie right before serving.