Step 1: Prepare the pie crust. Ensure your 9-inch pie crust is fully baked and cooled. If using a homemade crust, blind bake it until golden brown and set aside.
Step 2: Melt butterscotch base. In a medium heavy-bottomed saucepan, combine the packed light brown sugar, unsalted butter, and salt over medium heat. Cook, stirring constantly, until the butter is melted and the mixture comes to a gentle boil. Continue to boil for 2-3 minutes, stirring occasionally, allowing the sugars to caramelize slightly and develop a deeper flavor. Be careful not to burn it.
Step 3: Combine dairy. In a separate small bowl, whisk together 1/4 cup of the heavy cream and the whole milk.
Step 4: Create slurry. In another small bowl, whisk together the egg yolks and cornstarch until smooth and no lumps remain. This is your thickening slurry.
Step 5: Temper the eggs. Slowly stream about half of the hot butterscotch mixture from the saucepan into the egg yolk mixture, whisking constantly to temper the eggs. This prevents them from scrambling. Once combined, pour the tempered egg mixture back into the saucepan with the remaining butterscotch mixture.
Step 6: Cook the filling. Return the saucepan to medium-low heat. Cook, stirring constantly with a whisk, until the mixture thickens considerably and comes to a gentle boil. It should coat the back of a spoon. This will take about 5-8 minutes. Keep stirring to prevent scorching the bottom. Once thickened, remove from heat.
Step 7: Finish the filling. Stir in the remaining 1/4 cup of heavy cream and the vanilla extract until fully incorporated. The remaining heavy cream adds extra richness and silkiness. Pour the hot butterscotch filling into the cooled, pre-baked pie crust. Smooth the top with a spatula.
Step 8: Chill. Place a piece of plastic wrap directly onto the surface of the butterscotch filling to prevent a skin from forming. Refrigerate for at least 4 hours, or preferably overnight, until the pie is completely set and thoroughly chilled. This is crucial for the pie to hold its shape.
Step 9: Prepare whipped cream. Just before serving, in a large, cold bowl with cold beaters (or a stand mixer with a whisk attachment), beat the cold heavy cream, powdered sugar, and vanilla extract on high speed until stiff peaks form. Be careful not to overbeat, or it will turn granular.
Step 10: Assemble and serve. Remove the chilled pie from the refrigerator. Spoon or pipe the whipped cream topping generously over the set butterscotch filling. If desired, garnish with additional butterscotch drizzle, a sprinkle of sea salt flakes, or shaved chocolate. Slice and serve immediately. Store any leftovers in the refrigerator.