Soft Frosted Sugar Cookie Bars Recipe

Preheat the oven to 350ºF (177ºC). Line an 8" square baking pan² with parchment or foil. Set aside.
In a large bowl, toss together the flour, cornstarch, baking soda, and salt. Set aside.
1 and ½ cups (180g) all-purpose flour, 2 teaspoons (5g) cornstarch, 1 teaspoon baking soda, ½ teaspoon salt
In a medium size bowl, whisk together the butter and sugar until smooth. Whisk in the eggs and vanilla extract until completely combined.
½ cup (113g) unsalted butter, ¾ cup (150g) granulated sugar, 2 large eggs, 2 teaspoons vanilla extract
Pour the wet ingredients into the dry ingredients and stir until no flour clumps remain. If using, add the sprinkles and stir to combine.
⅓ cup (67g) sprinkles
Spread the cookie dough evenly into the prepared pan. It will be thick, so use your hands if you need to. 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 large bowl with a handheld mixer or a stand mixer fitted with the paddle attachment, beat the the butter on medium-high until smooth, about 2 minutes.
½ cup (113g) unsalted butter
Reduce the mixer to low, then slowly add the powdered sugar and mix until everything is combined. Add the vanilla extract and mix again.
2 cups (240g) powdered sugar, 1 teaspoon vanilla extract
Slowly add the milk or cream, watching for and stopping at desired consistency. Add the salt, then increase the mixer speed to medium and beat until completely smooth.
2 to 3 Tablespoons (30-45mL) milk or cream, pinch of salt
If after beating, the frosting is too stiff, add more cream. If it is too thin, add more powdered sugar, about 2 Tablespoons at a time. If using, add coloring³ when your frosting is at the desired consistency and stir with a spatula.
Using the foil or parchment overhang as handles, remove bars from baking pan, carefully remove bars from the lining, and place on a plate. Spread the buttercream evenly over the bars, top with more sprinkles (if desired), then slice to serve. Sugar cookie bars remains fresh covered tightly at room temperature for up to 5 days. Unfrosted or frosted bars freeze well, up to 3 months. Thaw in refrigerator overnight and allow to come to room temperature before decorating.