Spray 8" square baking dish with cooking spray. Line with parchment paper and spray the paper. Set aside.
Preheat oven to 350 degrees F.
In a medium mixing bowl, whisk together the dry ingredients ( flour, baking powder, soda and salt ).
In a medium saucepan brown 8 tablespoons of butter. Take of the heat and add brown sugar. Whisk until combined. Let cool slightly.
Add egg and whisk well.
Add vanilla extract and whisk again.
Add the dry ingredients all at once and fold in gently with a spatula. Do not over mix. The batter will be thick and sticky.
Spread in prepared baking dish.
Bake 18 to 20 minutes, but no longer than that. Let cool completely before frosting.
For the glaze: sift the confectioners' sugar into a small mixing bowl. Brown the remaining 6 tablespoons of butter. Add to sugar and whisk well. It may look crumbled. Add milk, one teaspoon at a time, until you reach drizzling but still thick consistency. Spread over cooled blondies. Cut into squares or triangles.
Notes
For me the magic number was to bake them for exactly 19 minutes. Let them cool completely before frosting and cutting.