Adjust oven rack to the middle position and preheat the oven to 350 degrees.
Spray a 13- by 9-inch baking pan with cooking spray or grease with butter.
In a medium bowl, whisk the flour, baking powder and salt together. Set aside.
In a second medium bowl, whisk the softened butter and brown sugar together until combined. Add the eggs and vanilla and mix well. Using a rubber spatula, fold the dry ingredients into the egg mixture until just combined.
Spoon the batter into the prepared pan, smoothing the top with a rubber spatula.
Bake until the top is shiny and cracked and feels firm to the touch, for 22 to 25 minutes.
Place the pan on a rack and let cool completely (about an hour).
Frosting:
Beat softened butter and cream cheese until well blended.
Add powdered sugar, vanilla, cinnamon, nutmeg and salt. Beat until creamy.
Frost blondies and cut into 1 ½ by 2-inch bars. Serve.