Preheat the oven to 375°F. Roll out the pastry and place it in a greased 9" pie plate. Flute the edges decoratively.
Melt the butter and set it aside to cool.
In a large bowl, mix together the flour, sugar, and salt.
Add the milk and eggs and beat well.
Stir in the vinegar, vanilla, and flavoring, if using, then the butter and nuts.
Pour the mixture into the crust and bake for 47 to 50 minutes. When done, the top will be puffed up and set, and the center should just barely wobble when you jiggle the pan.
Remove from the oven (the pie will finish setting up as it sits) and cool completely before slicing.