Preheat the oven to350 F. Line an 8-inch square pan with parchment paper and set aside.
Take a medium sized bowl and sift together the flour, baking powder, cardamom powder and salt.
In a large bowl beat the melted butter with white and brown sugar until combined well for 3 – 4 minutes. Add lemon zest, lemon juice, vanilla extract and the egg and beat again until combined.
Fold in the dry ingredients to the wet ingredients until just combined.
Fold in the blueberries gently into the batter.
Pour the batter into the prepared baking pan and place it onto the middle rack of the oven.
Bake for 30 – 35 minutes or until a tooth pick inserted comes out clean when inserted in the middle or center of the blondies.
Place the pan on the wire rack and let the blondies cool for at least 30 minutes before slicing and serving them.
Notes
Let the Butter Cool: before beating it with the sugar. If you will use hot or warm butter, it will melt the sugar and will scramble the eggs in the batter.Tossing the Blueberries in All-Purpose Flour: will help to disperse the blueberries evenly in the batter and will not let them sink in the bottom of the pan.