1. Preheat oven to 350°F and butter an 8-inch square pan. Spread pecans in an even layer on a baking sheet and bake until fragrant and toasted, about 6 minutes, shaking sheet once or twice. Transfer pecans to a cutting board to cool slightly, then chop.
2. In a small bowl, mix flour, baking powder and salt. In a medium bowl, stir butter and brown sugar until combined. Stir in egg and vanilla. Add flour mixture and stir until just combined. Stir in pecans. Spread batter evenly in baking pan. Bake until a tester inserted into center comes out clean, 22 to 24 minutes. Transfer to a wire rack to cool completely before cutting into squares.