Grandma's butterscotch was my brother's favorite pie when we went down to the country to visit. My favorite was chocolate, so she usually made both and had them sitting on the table waiting for us. I miss those days,
Place oven rack in middle position; preheat oven to 350 degrees F.
In the top of double boiler, combine brown sugar, flour and salt.
Add milk and stir until any lumps are dissolved.
Cook over boiling water (do not let top pan touch the water) on medium-high heat, stirring constantly, for 10 minutes or until very thick.
Remove from heat and add several spoonfuls of the hot mixture to the beaten eggs (about 1/2- 3/4 cup) and mix until smooth (this is called tempering the eggs; don't skip this step or you will end up with scrambled eggs in the filling!)
Add egg mixture to the pan and, stirring constantly, cook over boiling water until thickened (approximately 2-3 minutes.)
Remove from heat and stir in the butter and vanilla until thoroughly combined.
Let cool slightly for a few minutes, then pour into pie crust.
Make the meringue by beating egg whites until frothy.
Add cream of tartar and beat until stiff, but peaks still bend.
Gradually add sugar while beating; add vanilla and beat until combined and has stiff peaks.
Spread meringue over filling, making decorative swoops and swirls.
Bake at 350 degrees F for 15 minutes; remove and let cool and set up for at least 3 hours at room temperature or 1 hour at room temperature and 1 hour in refrigerator. Refrigerate leftovers.