Fit piecrust into a 9-inch pie plate, or start with a crust already in a foil plate. Brush with a little melted butter. Heat oven to 350 degrees F.
In a large bowl with an electric mixer, blend brown sugar, 3 tablespoons of the melted butter, flour and eggs on low speed.
Add corn syrup, molasses and vanilla and mix well.
Put 2 cups pecan halves in pie shell and pour mixture over pecans.
Bake at 350 degrees F for 50 to 55 minutes or until filling is set. Remove to wire rack and cool completely. Refrigerate for at least 1 hour before serving. Serve with a dollop of whipped cream, if desired. Makes 8 servings.