Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.
Cream the softened butter and granulated sugar in a stand mixer bowl until light and fluffy, approximately 3 minutes.
Add the egg and peppermint extract, beating until fully incorporated.
Sift together the flour, cocoa powder, baking soda, and salt in a separate medium bowl.
Gradually add the dry ingredients to the butter mixture on low speed until just combined.
Fold in the semi-sweet chocolate chips by hand.
Scoop rounded tablespoons of dough (approximately 1 inch in diameter) onto the prepared baking sheets, spaced 2 inches apart.
Bake for 8 to 10 minutes or until the edges are set but the centers remain slightly soft.
Remove from oven and immediately press crushed peppermint candy pieces into the tops of the warm cookies.
Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.