Ingredients
½cupunsalted butter,(115 grams), softened to room temperature
¾cupgranulated sugar,(150 grams)
1large egg,(plus 1 egg yolk)
2tsppure vanilla extract
1½cupsall purpose flour,(190 grams), spoon & leveled
¼tspsalt
1tspbaking powder
½tspbaking soda
1½tspcornstarch
½cuprainbow sprinkles,(80 grams), not nonpareils
¾cupunsalted butter,(170 grams), softened to room temperature
2¼cupsconfectioner’s sugar,(270 grams)
3tbspheavy cream,(45 milliliter), or half and half
1tsppure vanilla extract
salt,to taste
1food coloring,optional
Preparation
Preheat oven to 350 degrees F (177 degrees C). Line an 8-inch or 9-inch square baking pan with aluminum foil, 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 at low speed, slowly add the dry ingredients to the wet ingredients in 3 different parts.
Once combined, gently fold in ½ cup of sprinkles using a rubber spatula.
Press the cookie dough evenly into the prepared baking pan. Bake for 25 to 26 minutes or until very lightly browned on top.
Allow the bars to cool completely on a wire rack at room temperature before frosting.
With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy for about 2 minutes.
Add confectioners’ sugar, cream, and vanilla extract with the mixer running on low. Increase to high speed and beat for 3 full minutes.
Add more confectioners’ sugar if frosting is too thin or more cream if frosting is too thick. Add a pinch of salt if frosting is too sweet.
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 for up to 5 days.
