Preheat your oven to 350 degrees and grease a 9 by 13 pans
In a medium bowl, whisk the flour, baking powder, and salt together.
Cream together the sugar and butter, about two minutes. Then mix in the eggs and vanilla. Gently mix in the flour mixture.
Bake for 25 to 28 minutes, cool for 10 minutes, and then refrigerate for a half hour.
Mix together the maple syrup, butter, and powdered sugar for the maple frosting layer. Spread it evenly over the cooled blondies. It will be thin. Refrigerate for another half hour.
Microwave the white chocolate chips and butter together for in 30 second intervals, stirring in between until fully melted. Mixture will likely be chunky and not smooth. Add the milk, and that should smoothen it out. Spread it over the cooled maple frosting, and chill for another half hour before cutting.