• Preheat and Prepare the Pan
• Preheat the oven to 350°F. Line a 9x13 baking pan with parchment paper and spray it with non-stick cooking spray.

Make the Cookie Dough

• In a large mixing bowl, cream together the butter, sugar, brown sugar, and vanilla extract until the mixture is pale and fluffy (about 4–5 minutes).
• With the mixer on low, add the eggs one at a time, mixing until just combined.
• Add in the flour, baking powder, cornstarch, and salt. Mix until the dough comes together, about 2 minutes. Do not overmix, as this can make the bars tough.

Bake the Bars

• Spread the dough evenly into the prepared 9x13 pan. (Tip: Spray a piece of plastic wrap with non-stick spray and use it to press the dough into the pan smoothly.)
• Bake for 18–20 minutes, or until a toothpick inserted into the center comes out clean.
• Let the bars cool in the pan for 10 minutes. Then remove them and allow them to cool completely on a wire rack.

Prepare the Frosting

• In a mixing bowl, whisk together the butter and powdered sugar until smooth.
• Add the milk (or heavy cream) and vanilla extract and beat until the frosting is light and fluffy.

Assemble the Bars

• Once the bars are completely cool, spread the frosting evenly over the top.
• Decorate with your favorite Christmas sprinkles.
• Serve and Enjoy!
• Slice the bars into squares and serve.