Old Time Vinegar Pie
["3 Tbsp. vinegar", "1 c. sugar", "4 Tbsp. flour", "1 c. boiling water", "2 egg yolks", "1 Tbsp. butter"]
["Mix flour with half the sugar and add boiling water.", "Cook in double boiler for 15 minutes, stirring often.", "Add rest of sugar and beaten egg yolks; cook a few minutes longer.", "Add butter and vinegar.", "Flavor with lemon extract (if desired) and cool.", "Pour into baked shell and meringue."]
["vinegar", "sugar", "flour", "boiling water", "egg yolks", "butter"]