Grease a 13x9x2-inch baking pan; set aside. In a medium saucepan melt the 1 cup butter or margarine over low heat. Cool slightly. Stir granulated sugar and brown sugar into melted butter. Add eggs, one at a time, beating by hand just until combined. Stir in buttermilk or sour milk and 1 teaspoon vanilla.
Stir together flour and baking soda. Add flour mixture to egg mixture; stir just until combined. Divide batter into 2 equal portions (about 1-3/4 cups each). Stir pecans into one portion and spread this batter in prepared pan. Bake in a 350 degree F oven for 10 minutes. Stir the melted chocolate and cinnamon into the remaining batter; blend well. Spread the chocolate batter over partially baked brownies. Continue baking about 20 minutes or until toothpick inserted in center comes out clean. Cool.
Meanwhile, for sour cream frosting, in a medium mixing bowl beat together the sour cream, 1/4 cup butter or margarine, and 1 teaspoon vanilla with an electric mixer on low speed until smooth. Gradually add powdered sugar, beating on low speed until smooth and spreadable.
Frost cooled brownies with sour cream frosting or sift with powdered sugar; garnish with chocolate curls. Makes 36 bars.
*To make sour milk, place 1/2 teaspoon lemon juice in a glass measuring cup; add enough milk to make 1/4 cup liquid total.
**If you opt to simply sift powdered sugar over the brownies, lightly place a wire cooling rack over the brownies before topping with sugar; this adds an interesting design to the top of the brownies.