Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt until evenly blended.

In another large bowl, beat together the softened unsalted butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes.

Add in the egg, vanilla extract, and milk to the butter-sugar mixture and mix until well combined.

Gradually add the dry ingredient mixture to the wet ingredients and stir until a smooth dough forms, folding in the mini chocolate chips if using.

Scoop the dough into 1-inch balls and place them 2 inches apart on the prepared baking sheets.

Bake the cookies for 8-10 minutes, until the edges are set and the centers look slightly soft. Press a peppermint-striped chocolate kiss into the center of each cookie immediately after removing from the oven.

While the cookies are still warm, sprinkle crushed peppermint candies over the tops.