½cup(113g) unsalted buttersoftened to room temperature
2cups(240g) powdered sugar
2 to 3Tablespoons(30-45mL) milk or cream
SUGAR COOKIE CAKE
Preheat the oven to 350ºF (177ºC). Spray an 8" baking pan with non-stick spray. Set aside.
In a large bowl, toss together the flour, cornstarch, baking soda, and salt. Set aside.
In a medium size bowl, whisk together the butter and sugar until smooth. Whisk in the eggs and vanilla extract until completely combined.
Pour the wet ingredients into the dry ingredients and stir until no flour clumps remain. Pour in the sprinkles and stir to combine. Spread the cookie dough evenly into the prepared pan. Bake for 23-25 minutes until golden brown or a toothpick inserted in the center comes out mostly clean. You may cover the bars with foil to prevent browning, if preferred. Remove from oven and allow to cool on a wire rack completely before frosting.
In a medium bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, combine the butter, vanilla, and milk or cream. Turn the mixer onto low and slowly add the powdered sugar. Increase mixer speed to medium and beat until completely combined, scraping down sides as needed. Add salt to taste.
Using the foil overhang as handles, remove bars from baking pan, carefully remove foil, and place on a plate. Spread the buttercream evenly over the bars, then slice to serve. Sugar cookie bars remains fresh covered tightly at room temperature for up to 5 days. Unfrosted bars freeze well, up to 3 months. Thaw in refrigerator overnight and allow to come to room temperature before decorating.