Prepare the pie crust by pre-baking it according to package instructions or by making your own and baking it at 350°F for about 10-12 minutes until lightly golden. Allow it to cool completely.
In a medium saucepan, whisk together brown sugar, cornstarch, and salt. Gradually add whole milk while whisking. Cook over medium heat, stirring constantly for about 10-12 minutes until thickened. Remove from heat and stir in beaten egg yolks, unsalted butter, and vanilla extract.
Pour the warm butterscotch filling into the pre-baked pie crust and smooth the top. Let it cool at room temperature for about 30 minutes, then refrigerate for at least 2 hours to set.
Prepare the whipped cream by whipping heavy cream, powdered sugar, and vanilla extract until soft peaks form. Top the chilled pie with whipped cream before serving.