• Preheat oven to 350°F (175°C). Butter a 9×13-inch baking pan and line with parchment paper.
• In a medium bowl, whisk together the flour and salt. Set aside.
• Beat the butter and cream cheese until very smooth and creamy.
• Add the sugar and beat until light and fluffy.
• Add the egg and vanilla extract; mix until well combined, scraping down the bowl.
• Gradually add the flour mixture and mix just until incorporated.
• Spread the dough evenly into the prepared pan.
• Bake for 18–22 minutes, until the edges are set and the center is no longer wet. Do not overbake.
• Cool completely in the pan on a wire rack before frosting.

Make the Frosting

• Beat the butter and cream cheese until smooth and creamy.
• Gradually add the powdered sugar, mixing on low speed.
• Add the cornstarch and mix until fully incorporated.
• Add the vanilla extract, salt, and 1–3 drops of food coloring, if desired; mix until smooth and evenly colored.
• Beat on medium-high speed for 1–2 minutes until light and fluffy.
• Spread frosting evenly over cooled cookie bars.
• Top with sprinkles, slice, and serve.