Whisk together the flour, cornstarch, baking powder, and salt. Set aside and preheat the oven to 350° F.

In a medium sized bowl, whisk together the melted butter and sugar until combined.

Add the eggs and vanilla extract. Whisk til smooth and glossy, about 30-60 seconds.

Add the dry ingredients to the wet, and gently fold until just combined and no streaks of flour remain. Do not overmix.

Pour the cookie dough into the prepped 8x8” baking dish. Use a rubber spatula to spread into an even layer.

Bake at 350° F for 20-25 until the edges are just browning, and a toothpick inserted comes out with fudgy crumbs, not dry.  Do not overbake.

Remove the cookie bars from the oven and allow to cool completely before frosting.

With an electric mixer, beat the softened butter on med-low speed for 30 seconds until smooth. Turn the mixer down to low speed.

Add the powdered sugar, milk, salt, and vanilla. Mix on low speed until combined, then turn the speed up to medium and whip the frosting until light and fluffy for 5 minutes.

If you're adding food coloring, turn off the mixer and add in your colors. Whip and add more color if needed to get the shade you want.

Use the parchment paper sling to remove the dessert bars from the pan. Spread the frosting on the cooled cookie bars and top with festive sprinkles. Cut the bars and serve.