Heat oven to 350°F. In large bowl, beat granulated sugar, 1/2 cup brown sugar, 1/2 cup butter, 1 teaspoon vanilla and the eggs with electric mixer on medium speed, or mix with spoon, until light and fluffy. Stir in flour, baking powder and salt. In ungreased 13x9-inch pan, spread batter evenly.
Bake 20 to 23 minutes or until golden brown and toothpick inserted in center comes out clean. Cool completely, about 45 minutes.
In 2-quart saucepan, melt 1/3 cup butter over low heat. Stir in 2/3 cup brown sugar; cook over low heat 2 minutes, stirring constantly. Stir in milk; cook until mixture comes to a rolling boil. Remove from heat. Gradually stir in powdered sugar and vanilla, mixing well with spoon after each addition, until smooth and spreadable. If necessary, add more milk, a few drops at a time. Spread frosting over brownies. Immediately sprinkle with pecans. For brownies, cut into 6 rows by 6 rows.