In a large saucepan, combine sugar, flour, a pinch of salt and the egg yolks. Over medium heat, add about 1/4 cup of the cream, stirring until combined and turns into a smooth thick paste. Gradually add the remaining cream, stirring constantly until combined. Add butter and peanut butter. Keep stirring continuously until mixture begins to thicken.
Reduce heat, continue stirring and allow to cook slowly until mixture is a thick pudding consistency and the peanut butter is fully incorporated. Remove from heat and stir in vanilla. Allow to cool for about 15 minutes.
Pour peanut butter custard into the baked and cooled pie crust. Preheat oven to 400 degrees F.
To prepare meringue, beat egg whites with cream of tartar until frothy. Beat in sugar (a little at a time) until stiff and glossy. Spread over top of peanut butter custard and bake pie for 8 to 10 minutes until meringue is lightly browned. Cool and store in refrigerator.