Mix 2 cups flour, baking powder, and 2/3 cup brown sugar. Cut butter into mixture with pastry blender until well blended. Mixture will appear dry. Pat evenly into bottom of well greased 11x7 1/2 or 9x9x1 3/4-inch pan. Bake 15 minutes in 350° oven.
Mix 1/2 cup brown sugar and 1/3 cup flour together. Beat eggs and add to mixture. Add corn syrup, salt, and vanilla. Pour over partly baked layer. Sprinkle with chopped pecans. Bake in 350° oven until set; about 40 minutes. Cut into bars while warm.