Best-Ever Butterscotch Cookies

1 tablespoon vinegar
1 cup (about) evaporated milk
1/2 cup butter or regular margarine
1 1/2 cups brown sugar -- firmly packed
2 eggs
1 teaspoon vanilla
2 1/2 cups sifted flour
1 teaspoon baking soda
1/2 teaspoon baking powder
1/2 teaspoon salt
Walnuts or pecan halves
Brown Butter Frosting:
1/2 cup butter
2 cups sifted confectioners sugar
2 to 4 tablespoons boiling water

Put vinegar in a 1-cup measure; add evaporated milk and set aside. Beat butter until light; add brown sugar and beat until mixture is light and fluffy. Beat in eggs and vanilla to blend thoroughly. Sift together flour, baking soda, baking powder and salt. Stir evaporated milk and add alternately with dry ingredients to creamed mixture. Stir in chopped nuts. Drop rounded tablespoonfuls of dough about 2-1/2-in. apart onto lightly greased baking sheet. Bake in moderate oven (350') 10 to 12 minutes, or until lightly browned and barely firm to touch. Remove cookies and cool on racks. When cool, spread with Brown Butter Frosting and press a walnut or pecan half in each cookie.

Brown Butter Frosting: Melt butter in small saucepan and cook over medium heat, stirring constantly, until butter stops bubbling and is nut-brown in color (do not scorch). Combine with confectioners sugar and boiling water; beat until smooth and of spreading consistency. (Makes enough to frost about 5 dozen cookies)