Preheat oven to 350°F. Line 13x9x2-inch metal baking pan with parchment paper; coat with nonstick spray. Combine 6 tablespoons butter, 4 tablespoons brown sugar, and 2 tablespoons sugar in medium bowl. Using electric mixer, beat mixture until light and fluffy, occasionally scraping down sides of bowl, about 2 minutes. Add egg; beat until pale and fluffy. Add oats, flour, baking powder, baking soda, and salt and beat until well blended, about 1 minute. Turn oat mixture out onto prepared baking pan; press out evenly to edges of pan. Bake until light golden on top, 17 to 18 minutes. Transfer baking pan to rack and cool cookie completely.