Whisk all-purpose flour, baking powder, and salt in a bowl and set aside.
In a large mixing bowl, beat butter and sugar on medium speed until light and fluffy, about 2-3 minutes. Scrape down the sides the bowl in between, to ensure the butter and sugar are fully incorporated.
Add egg, milk, vanilla and peppermint extracts and beat on medium speed until well combined.
Gradually add the dry ingredients to the butter mixture in two additions, mix on low speed just until no flour streaks remain. Fold gently with a spatula to avoid overworking the dough. Cover and chill the dough for about 30 to 45 minutes.
When ready to bake, preheat oven to 350° F (180° C). Line baking sheets with parchment paper or silpat.
Shape dough into 1-inch balls using a measuring tablespoon or cookie scoop. Roll each cookie dough ball in sugar/sanding sugar and place on the prepared baking sheet about 2 inches apart.
Bake at 350° F (180° C) for 9 to 10 minutes or until the edges are set, the centres are still soft and tops look slightly cracked.
Remove from the oven and allow to cool down for about a minute.
Gently press a candy kiss into the center of each warm cookie. Leave them on the tray for about 5 minutes, then transfer to a wire rack to cool completely so that the kisses set.