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).
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.