1. Preheat oven to 350ºF. Line an 8-inch square baking pan with heavy-duty foil, leaving at least a 1-inch overhang on all sides.
2. In a small bowl, combine flour, baking powder and salt. Melt butter in a medium saucepan over low heat. Stir in brown sugar until moistened. Let cool slightly. Off heat, whisk in egg and vanilla. Stir in flour mixture until just combined. Stir in nuts.
3. Scrape batter into baking pan, smoothing top with a spatula. Scatter berries over batter. Bake until blondies are set in center, about 30 minutes. Cool completely in pan on wire rack.
4. Grasping overhanging foil on opposite sides of pan, lift out blondies and place on cutting board. Cut into 16 squares and serve.
Note: Nutritional analysis is per blondie.