Preheat oven to 350°F (180°C). Line a baking tray with parchment paper. Set aside.
Whisk flour, baking soda, and salt together in a medium sized bowl. Set aside.
In a large bowl, beat butter and sugar together until well combined. Mix for at least 2 minutes.
Add egg, vanilla, and milk, and mix until combined, scraping down the sides of the bowl as needed.
Add dry ingredients in, and mix until just combined. Roll out 1" balls of cookie dough, roll in coloured sanding sugar, and place on prepared baking tray. I fit 12 per tray.
Bake for 10-12 minutes, then remove from oven and let cookies sit for 3 minutes. Then add a Hershey Kiss to the middle of each cookie. Press them in lightly.
Transfer cookies to a wire cooling rack to cool to room temperature. Store in an airtight container at room temperature for up to 5 days.