PREP: Preheat the oven to 350 degrees F. Grease a 9x13-inch pan with cooking spray and line with parchment paper (I don't recommend lining with foil). These bars are sticky so you do want the parchment paper.
WETINGREDIENTS: In a large bowl, combine 12 tablespoons room-temperature butter with the 1 cup white sugar and 1/2 cup light brown sugar in a medium sized bowl. Beat with hand mixer (or in a stand mixer) until light and creamy. Add in the egg, egg yolk, and 2 teaspoons vanilla. Mix until combined.
DRY INGREDIENTS: In a separate large bowl, combine the 2 cups + 2 tablespoons white flour, 1/2 teaspoon baking soda, and 1/2 teaspoon salt. Stir. Add wet ingredients to dry ingredients and stir until just combined. With a wooden spoon, stir in 1 cup of white chocolate chips and stir until combined.
BAKE: Press the cookie dough into the prepared 9x13-inch pan. Bake the bars for 22-26 minutes or until the top is no longer glossy and edges are ever-so-slightly browned. Remove from the oven and immediately top with the miniature marshmallows. Return to the oven for 1 minute and 30 seconds.
FINISHING: Gently press the marshmallows down a little. Sprinkle the remaining 1/4 cup white chocolate chips evenly over the bars. Sprinkle the 3/4 cup M&M's evenly over the bars. Scatter the sprinkles evenly over the bars.
SERVE: Let cool completely (these bars take 1-3 hours to really set up) and then cut with a hot, sharp knife (run the knife under hot water, dry, make a slice, repeat). Confetti Bars are best enjoyed within 1-2 days; after that the marshmallows become unpleasant.
Note 1: Use 1 tablespoon nonpareil sprinkles or 2-3 tablespoons jimmies sprinkles.