Preheat the oven to 350°F (175°C). Grease an 8x8 inch baking pan with cooking spray or butter.
In a mixing bowl, beat the softened butter and granulated sugar together until light and fluffy. Add in the egg and vanilla extract, and beat until well combined.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the butter mixture, stirring until just combined.
Fold in the fresh blueberries and chopped walnuts, being careful not to crush the blueberries.
In a small bowl, mix together the brown sugar and ground cinnamon.
Pour the blueberry and walnut mixture into the prepared baking pan and sprinkle the cinnamon sugar mixture on top.
Bake for 35-40 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean.
Allow the blueberry walnut dessert bars to cool completely in the pan before slicing and serving.