Butterscotch Pie

0 In a saucepan, melt butter on medium heat. 
1 Remove from heat and add flour and stir until smooth. Then stir in brown sugar.  
2 Returning the sauce pan to the heat stir in milk and salt until incorporated.  
3 Cook the brown sugar mixture, stirring often, until it has thickened and is bubbly. 
4 Turn the heat to low and cook an additional 2 minutes. Remove from heat.
5 Remove 1 cup of filling and stir into egg yolks. This tempers the yolks so they don't scramble.
6 Add yolk mixture back to the pan and stir constantly. Bring to a gentle boil and cook for 2 additional minutes. 
7 Remove saucepan from heat and stir in vanilla.
8 Pour the butterscotch mixture into the par baked pie crust.
Meringue
10 In a stand mixer beat egg whites and cream of tartar on medium speed until soft peaks form. 
11 Turn mixer up to high and add in sugar 1 tablespoon at a time. Mix until stiff glossy peaks are formed. 
12 Spread meringue over butterscotch filling. Make sure to spread meringue to the edges to seal the crust. 
13 Bake at 350º F for 12-15 minutes until meringue is golden brown. Cool for 1 hour before placing in the fridge. Refrigerate for 3 hours before serving.