Step 1: Prepare Your Workspace
Pre-bake your pie crust according to package or recipe directions and let it cool completely. Have all your ingredients measured and ready before you start cooking—custard waits for no one!

Step 2: Combine Dry Ingredients
In your heavy-bottomed saucepan, whisk together the brown sugar, flour (or cornstarch), and salt until no lumps remain. Breaking up the brown sugar clumps now will save you headaches later.

Step 3: Add Milk Gradually
Pour in about 1 cup of the milk and whisk until you have a smooth paste with no lumps. Then gradually whisk in the remaining 2 cups of milk until everything is completely combined and smooth.

Step 4: Cook the Custard
Place the saucepan over medium heat and cook, whisking constantly, until the mixture comes to a full boil. This will take 8-12 minutes. The mixture will gradually thicken, and you’ll notice it starting to bubble. Once it reaches a rolling boil, let it boil for 1 minute while whisking continuously, then remove from heat.

Step 5: Temper the Egg Yolks
In a separate bowl, have your beaten egg yolks ready. Slowly pour about 1 cup of the hot butterscotch mixture into the yolks while whisking constantly. This gradually raises the temperature of the yolks without scrambling them. Once combined, pour the egg yolk mixture back into the saucepan with the remaining butterscotch filling.

Step 6: Final Cook
Return the saucepan to medium heat and cook, whisking constantly, for 2-3 minutes more until the filling is thick and glossy. It should coat the back of a spoon and hold its shape when you draw a line through it.

Step 7: Add Butter and Vanilla
Remove from heat and stir in the butter pieces and vanilla extract until the butter is completely melted and incorporated. The butter adds incredible richness and shine.

Step 8: Strain (Optional)
For an ultra-smooth filling, pour the custard through a fine-mesh strainer into a clean bowl. This removes any small lumps or cooked egg bits.

Step 9: Fill the Pie
Pour the hot butterscotch filling into your cooled pie crust, spreading it evenly with a spatula. Press a piece of plastic wrap directly onto the surface of the filling to prevent a skin from forming.

Step 10: Chill
Refrigerate the pie for at least 4 hours or overnight until the filling is completely set and cold.

Step 11: Add Topping
For whipped cream: Beat heavy cream with powdered sugar and vanilla until stiff peaks form. Spread or pipe over the chilled pie just before serving.

For meringue: Beat egg whites with cream of tartar until soft peaks form. Gradually add sugar and beat until stiff, glossy peaks form. Spread over the pie, making sure to seal the edges. Brown under the broiler for 1-2 minutes, watching carefully, or use a kitchen torch.

Step 12: Slice and Serve
Use a sharp knife dipped in hot water (and wiped dry between cuts) to slice clean pieces. Serve chilled and enjoy!