Preheat the oven to 350 degrees F. Spray a 9x13 pan with cooking spray. You may also line the pan with aluminum foil or parchment paper and spray with cooking spray.
In a medium bowl, whisk together baking powder, salt and flour. Set aside.
Using an electric mixer, cream together the granulated sugar, brown sugar and butter. Add the eggs, vanilla and butter extract and beat to combine. Add the dry ingredients and mix until combined. Stir in the sprinkles.
Press the dough into your prepared pan. It's going to be very sticky, and that's just what you want! Spray your hands with cooking spray so the dough won't stick to your hands.
Bake the blondies for 22 to 24 minutes. They'll be slightly golden but just a little jiggly in the middle. They will set as they cool. Allow the blondies to cool completely before frosting.
To make the frosting:
Using an electric mixer, cream the butter until smooth. Slowly add in the powdered sugar and mix until crumbly. Add in the vanilla and milk and mix until smooth. Frost the blondies. Top with additional sprinkles.
Store covered at room temperature or in the refrigerator for up to 5 days.
Recipe by Kitchen Meets Girl at https://kitchenmeetsgirl.com/cake-batter-blondie-bars/