Preheat the oven to 375 degrees F and line a 9x13 baking dish with parchment paper or aluminum foil.
In a large bowl, combine the flour, salt, baking powder, and cornstarch.
Mix the softened butter and sugar with an electric mixer until smooth and fluffy. This should take about 2-3 minutes. Add the eggs one at a time along with the vanilla extract.
Add the flour mixture to the sugar mixture and mix until well combined. Fold in the sprinkles.
Press the cookie dough evenly into the prepared baking dish. Bake for 25-27 minutes. Allow the cookie bars to cool completely.
While the bars cool, make the frosting by placing the butter into a large bowl. With an electric hand mixer, add in the powdered sugar a little at a time until fully mixed. Add the vanilla and milk, stir it in until combined then place the mixer on medium-high speed and whip for 3 minutes until light and fluffy.
Place about ½ cup of the frosting in a small bowl. Color the larger amount of frosting with the green and yellow food coloring until it reaches your desired color. Add red food coloring to the smaller bowl of frosting.
Once the cookie is cool, take it out of the baking dish and frost the top with the green frosting. Place the red frosting into a piping bag fitted with a number 12 round tip. You can cut the bars first then pipe on the hearts or you can cut after, I prefer cutting before so I don’t mess up any of the heart decorations accidentally. Add the hearts and serve.