Preheat the oven to 375°F. Line a 9x13 baking dish with parchment paper or foil and spray with cooking spray, set it aside. In a large bowl stir together the flour, baking powder, and salt, set aside.
In a large bowl with an electric hand mixer or the body is a stand mixer with the paddle attachment cream the butter with the sugar until light and fluffy, 2 minutes.
Add the eggs one at a time, letting the first egg mix all the way in before adding the next. Add the vanilla and mix it in until combined.
Next add the flour mixture and stir it in until combined, scrape down the sides, and mix it in a few more times until fully mixed in.
Lastly add the sprinkles and stir them in.
Press the cookie dough into the prepared baking dish until even. I like to wet my hands slightly and press it in, it makes it easier. Bake for 25-28 minutes until lightly golden and a toothpick inserted into the center comes out clean.
Place on a wire rack 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 salt, 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 bars are cool, take them 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.