Preheat oven to 375°F (190°C) and grease an 8-in. (2 L) square cake pan. To make streusel topping, combine flour, pecans, brown sugar and cinnamon in a small bowl. Using cool fingertips, mix in butter until mixture resembles coarse meal. Set aside.
To make the squares, melt butter over medium heat, cooking until light brown and fragrant, 6 to 7 min.; cool slightly. Using an electric mixer, cream together cooled butter and both sugars. Add egg and vanilla, beating until light and fluffy. In a separate bowl, sift together flour, baking powder and salt. Add the flour mixture and milk to the egg mixture alternately, mixing at low speed until just combined.
Spread batter evenly in prepared pan. Scatter blueberries overtop and sprinkle evenly with streusel topping. Bake for 45 to 50 min. or until a toothpick inserted into the centre of the pan comes out clean. Let cool to room temperature before cutting into 16 squares. Serve or store in an airtight container for up to 5 days.