This is the authentic recipe for the original 1930s trefoils sold by the Girl Scouts.  It is also the recipe used by the old Omar Baking Co. in Milwaukee, which was one of the first commercial suppliers of Girl Scout cookies. The recipe appeared in the Milwaukee Journal in 1993.

Omar Baking Co. Girl Scout Cookies
Ingredients:
1 c. butter
1 c. sugar
2 eggs, beaten
2 tbsp. milk
2 tsp. baking powder
3 c. flour
2 tsp. vanilla
¼ tsp. salt

In mixing bowl, cream together butter and sugar. Stir in beaten eggs. Add remaining ingredients and blend well. Wrap dough and chill thoroughly. Roll out dough on floured and sugared pastry cloth. Cut into circles about 3″ in diameter. Place on greased baking sheet. Bake at 350° F. until lightly browned, about 11 – 14 min. Makes 3 ½ dozen.