In a medium-size bowl, beat together the egg, sugar, oil and vanilla until fluffy.
In a small bowl, combine the flour, baking powder and salt. Whisk until well combined.
Add the dry ingredients alternately with the milk. Mix just until combined.
Fold in the blueberries.
Spray an 8" or 9" square pan with cooking spray.
Pour the batter into the prepared pan.
For the topping, combine the brown sugar, flour, cinnamon, butter and nuts. Mix well. Sprinkle the topping over the cake batter.
Bake for 25-35 minutes. If the pan is a 9" pan, it will take closer to 25 minutes. If it is an 8" pan, it will be done in closer to 35 minutes.
The calories shown are based on the cake being cut into 12 pieces, with 1 serving being 1 piece of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.