Preheat oven to 325ºF. Grease and line an 8-inch square baking pan. Set aside.
Combine the flour, sugar and salt in a mixing bowl. Cut in the butter, until the mixture resembles chunky, wet sand.
Pour the cookie mixture into the prepared pan and distribute evenly. Then, using your fingers or a fork, press down on the dough to gently compact it.
Cook the dough on the middle rack of the preheated oven for 50-55 minutes, until lightly browned, and a bit darker around the edges.
Let cookies cool in the pan for at least 30 minutes. When cookies have cooled, remove from pan onto a cutting board. Trim off the edges, cut into approximately 49, 1-inch cubes. Once cut into cubes, refrigerate shortbread at least 1 hour.
Meanwhile, make the frosting. In one bowl, combine powdered sugar, water and vanilla. Then, divide the vanilla frosting, and add gel food coloring until the desired color is reached (recommend 6-8 drops of food coloring to color half of the frosting).
Now for the fun part! Enrobe the shortbread cookies in frosting and set on a wire rack. There, top the frosted cookies with crushed mint candy or matching colored sprinkles. Transfer to the freezer for at least an hour, ideally in an air tight container to prevent any frosting from melting in the humidity. Alternatively, have fun frosting and topping the cookies with different colors, candies and sprinkles, and enjoy!