For the blondie bars: Preheat the oven to 350 degrees F. Spray a 9-by-13-inch pan with cooking spray, or line with foil and spray with cooking spray for easy cleanup.
In a medium bowl, whisk together the baking powder, salt, and flour. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the granulated sugar, brown sugar, and butter. Add the eggs, vanilla, and butter extract, and beat until combined. Add the dry ingredients, and mix until combined. Stir in the sprinkles.
Press the dough into the prepared pan. (The dough will be sticky; spray your hands with cooking spray to keep it from sticking to you.) Bake for 22 to 24 minutes. The bars will be slightly golden but still have the slightest jiggle in the center. They'll set as they cool. Let cool completely before frosting.
For the frosting: Using an electric hand mixer or a stand mixer fitted with the paddle attachment, beat the butter until smooth. Slowly add the powdered sugar, and mix until crumbly. Add the vanilla and milk, and mix until smooth. Frost the bars. Top with sprinkles, and cut into bars. Store, covered, at room temperature or in the refrigerator. Best eaten at room temperature within 5 days.
You can find butter extract at most grocery stores in the baking aisle.
"Dessert Mashups: Tasty Two-in-One Treats Including Sconuts, S'morescake, Chocolate Chip Cookie Pie and Many More" by Dorothy Kern Copyright Ulysses Press 2014. Reprinted with permission from Ulysses Press. All rights reserved.