1. Preheat oven to 350 degrees F. Prepare an 8 or 9-inch square baking pan by lining with aluminum foil or parchment. Spray the foil or parchment with nonstick cooking spray.
2. Combine the flour, baking powder, baking soda, and salt in a small bowl and whisk to combine.
3. In a mixing bowl, combine sugar and melted, cooled butter and whisk until the butter is absorbed into the sugar. Add egg and egg yolk and whisk until combined.
4. Add the dry ingredients and stir to combine with a wooden spoon.
5. Press into prepared baking pan. Bake until just set on top and the edges are turning golden, about 30 minutes. Don’t overbake! They will look undercooked but will firm up after cooling.
6. Let cool completely before frosting.

Make the frosting:

1. With a hand mixer or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, cream, vanilla, and salt with the mixer running on low. Increase the speed to high and beat until the frosting is light and fluffy, scraping down the sides halfway through, about 4 minutes. Add more cream if needed to thin the frosting.
2. Spread the frosting on the cooled bars, smoothing it out with a spatula. Decorate with sprinkles, if using. Cut into 16 bars. Store in an airtight container at room temperature for 3 days or in the refrigerator for 5 days.