Preheat the oven to 350 and prepare an 8x8 baking pan with grease or pachment . Set to the side.
In a medium-sized bowl, combine the flour, baking powder, baking soda, cornstrach, and salt. Mix to combine and set to the side.
In a large bowl with a hand mixer or in a stand mixer with a paddle attachment, cream the butter with the sugar. Mix the two together for about 2-3 minutes, until light and fluffy. Scrape down the bowl and add in the egg and egg yolk, mixing to combine. Add in the extracts and mix to combine.
Using a spatula or wooden spoon, fold in the sprinkles. Make sure they are evenly divided.
Add the sugar cookie bar dough to the pan and bake for 30 minutes, or until golden brown.
While the bars are baking, make the frosting. In a large bowl with a hand mixture or in a stand mixer with a paddle attachment, mix the butter, powdered sugar, heavy cream, vanilla, and salt. Taste the frosting as you go along and adjust sweetness as needed by adding in more powdered sugar. Start with less and you can always add more. If using food coloring or sprinkles, add them in at the end. If the frosting is rather stiff, add in another teaspoon of heavy cream. Frost the bars when fully cool.
