Preheat the oven to 350 degrees F. Line a 9x13-inch baking dish with aluminum foil. Spray the foil with cooking spray and set aside.

Whisk the flour, baking powder, and salt together in a medium-sized bowl. Set aside.

Cream the softened butter and brown sugar together in the bowl of a stand mixer until light and fluffy. Add the egg, egg yolk, and vanilla extract. Beat until combined. Add the flour mixture, a little at a time, until well combined. Scrape down the sides of the mixing bowl as needed.

Add the mini chocolate chips, peanut butter chips, and sprinkles. Beat until all of the mix-ins are distributed throughout the dough.

Press the dough into the prepared baking dish in an even layer. Top with extra mini chocolate chips and sprinkles. Lightly press the Reese's Pieces eggs into the dough, distributing them randomly over the surface.

Bake for 20-25 minutes, or until a toothpick, when inserted into the center, comes out clean. Allow the blondies to cool completely before cutting them into bars.