Preheat the oven to 350℉. Line 2 cookie sheets with parchment paper and set aside.
In a large bowl, cream together the butter and sugar until light and flurry. Mix in the egg, vanilla and peppermint extract until fully combined.
In a separate bowl, mix together the flour, baking soda and salt.
Alternate between mixing in the dry ingredients and the milk into the wet ingredients until fully incorporated.
Pour the sanding sugar into a small, shallow bowl.
Using a medium cookie scoop, scoop the cookie dough into your hands and form into a ball. Roll the dough in the sanding sugar until fully coated. Place on the prepared cookie sheet. Repeat with the remaining dough.
Place the cookie sheets in the preheated oven for 8-10 minutes, or until the middle of the cookie dough is no longer wet. Remove from the oven and allow to cool completely on the cookie sheets.