Preheat the oven to 350 degrees Fahrenheit and line a baking sheet with parchment paper.
In a large bowl, beat together the butter and sugar until light and fluffy.
Add the egg and vanilla extract and beat until smooth.
Add the flour, baking soda, and salt to the bowl and mix until smooth.
Pour in the milk and mix until fully incorporated.
Add the red and green sugar crystals and carefully fold them into the dough.
Use a cookie scoop to scoop out the dough and roll it into a ball between your hands. Place it on the lined baking sheet. Continue rolling until all the dough has been formed into balls.
Bake the cookies in the preheated for 12 to 15 minutes, until the edges of the cookies are just beginning to brown.
Press one Kiss into the top of each cookie immediately after removing them from the oven.
Allow the cookies to cool on the baking sheet for five minutes, then transfer them to a wire cooling rack to cool completely before serving.