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.

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.

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.

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.

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.