Preheat your oven to 350°F (175°C). Grease a 9×9-inch square baking pan or line it with parchment paper, leaving an overhang on the sides for easy removal later.
In a large mixing bowl, beat the softened butter and granulated sugar together until the mixture is light and fluffy. This usually takes about 2 to 3 minutes with an electric mixer.
Add the egg and vanilla extract to the butter mixture. Beat again until everything is well combined and smooth.
In a separate small bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until a soft dough forms. Do not overmix.
Press the cookie dough evenly into the bottom of your prepared pan. Bake for 18 to 20 minutes, or until the edges are very lightly golden. The center should still look soft. Remove from the oven and let them cool completely in the pan.
While the bars cool, make the frosting. Beat the 1/2 cup of butter until smooth. Add the powdered sugar, milk, and your choice of food coloring. Mix until the frosting is creamy and reaches your desired color intensity.
Spread the colorful frosting over the cooled cookie bars. While the frosting is still wet, press the candy eyeballs into the top. Use different sizes and placements to give them a “monster” look. Slice into 16 squares and serve.