Combine first 4 ingredients in a large bowl; add butter, eggs, and vanilla, stirring until blended. Stir in pecans.
Spread into a greased and floured 15- x 10-inch jellyroll pan.
Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out clean. Cool on a wire rack. Spread with frosting. Cut brownies into 1-inch squares.