For the bars: Preheat oven to 350F degrees. Line an 8-inch or 9-inch square baking pan with parchment paper, making sure to leave enough overhang on the sides. Set aside.

In a large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, cream the softened butter for about 1 minute on medium speed. Get it nice and smooth, then add the sugar on medium speed until fluffy and light in color. Beat in egg, egg yolk, and vanilla. Scrape down the sides as needed.

In a medium bowl, whisk together the flour, salt, baking powder, baking soda, and cornstarch. With the mixer running on low speed, slowly add the dry ingredients to the wet ingredients in 3 different parts. The dough is quite thick. Once combined, gently fold in 1/2 cup sprinkles using a rubber spatula (or by mixing on low speed).

Press the cookie dough evenly into the prepared baking pan. Bake for 25-26 minutes or until very lightly browned on top. (My bars puffed up while baking, then collapsed and cracked a bit on top. It did not seem to affect the overall yumminess). You want the cookie bars to be extra soft, so careful not to overbake. Allow the bars to cool completely on a wire rack at room temperature before frosting.

For the frosting: If you don't want a super thick layer of frosting as pictured, halve the frosting recipe. With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy - about 2 minutes. Add confectioners' sugar and beat for another 2-3 minutes until fully incoroporated. Scrape down as needed. Then add in the cream, and vanilla extract with the mixer running on low. Increase to high speed and beat for 8 full minutes. Add a pinch of salt if frosting is too sweet. I usually add anywhere between 1/8 - 1/4 teaspoon of salt. Beat in food coloring, if using.

Remove cooled bars from the baking pan using the aluminum foil overhang on the sides. Frost the bars with a thick layer of frosting and decorate with extra sprinkles if desired. Cut into squares. Use a very sharp knife and wipe off with a paper towel between each cut to make neat, even cuts.

Store bars in an airtight container at room temperature for up to 2 days and in the refrigerator up to 5.

* To soften your butter quickly, cut into small pieces, and let sit out for 30-45 minutes. Go catch up on your favorite television and when you come back the butter should be perfectly softened ready to go.

*Room temperature egg and egg yolk preferred. To bring the egg to room temperature quickly, place in a warm glass of water for about 10 minutes as you get the other ingredients ready. Or just set out when you set out the butter to soften 1 hour ahead of time.

*Please use regular sprinkles (aka "jimmies") in the cookie bar base. Nonpareils (the little balls) will bleed their color and turn the sugar cookie dough green/purple/brown. I order my sprinkles in bulk here.

*Heavy Cream or half-and-half is preferred for creamiest, fluffiest frosting. I have used milk when in a pinch and it still came out delicious!

*You can try a different size pan. I originally tried putting it in a 9x13 pan but wasn't happy with the thickness of the bar and went back to my trusty 8x8 pan. Baking time would be less if in a bigger pan.