Place the brown sugar, heavy cream, and butter into a medium saucepan. Place over medium-low heat, and start whisking immediately so the sugar doesn’t burn. Once the butter melts it will become smooth. Stirring occasionally, bring it to a boil. Then reduce to a simmer and cook for 2 minutes until slightly thickened. Remove from the heat.
Reserve 2 tablespoons of milk. Pour the rest of the milk into the saucepan and whisk to combine. Set aside.
Place a fine-mesh strainer over a large bowl. Set aside.
In a medium bowl, whisk together the reserved milk, cornstarch, egg yolks, and salt until smooth. Stream the egg mixture into the saucepan while whisking. Place the pot back on the stove over medium-low heat, whisking constantly so it doesn’t scorch on the bottom. The mixture will thicken to a thick pudding-like consistency.
Immediately pour some of the custard into the mesh strainer over the bowl. (You will need to do this a little at a time.) Use a rubber spatula to press the custard through the strainer. Don’t forget to scrape the custard off the bottom of the strainer when you are done.Pro Tip: Straining the custard will make sure that is completely smooth. There is a chance the egg may cook slightly and leave a few small chunks, which is why we strain. If you don’t mind that possibility, you can skip this step entirely.
Whisk the vanilla into the custard. Pour the butterscotch custard into the baked pie crust and smooth out the top. Let cool to room temperature, about 30 minutes to 1 hour.
Cover the pie with plastic wrap. Place in the fridge for at least 6 hours until set (chilling overnight is best). Cut into 8 slices and serve with whipped cream and your favorite toppings.
Notes
You need to bake and cool the pie crust ahead of time. This can be done up to 1 day ahead of time. Store, covered, on the counter at room temperature.
If you want to make butterscotch cinnamon pie, add 1 ½ teaspoons of ground cinnamon to the egg and cornstarch mixture.