In a large bowl, combine the flour, sugar, baking soda and salt. In a small bowl, combine eggs and sour cream. In a small saucepan, cook and stir water, butter and chocolate just until melted. Stir into dry ingredients. Stir in sour cream mixture until well blended.
Pour into a greased 15x10x1-in. baking pan. Bake at 375° for 25 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack for 30 minutes.
Meanwhile, for frosting, place confectioner's sugar in a large bowl; set aside.
In a small saucepan combine the butter, milk and chocolate. Bring to a boil, stirring constantly. Beat into confectioner's sugar until smooth. Stir in vanilla. Let stand for 5 minutes. Spread over warm bars. Top with pecan halves. Cool completely before cutting into bars.