Heat the oven to 350°F (175°C) and line baking sheets with parchment paper. The parchment helps prevent peppermint candy from sticking to the pan and makes cleanup easier.

Add the cake mix, eggs, vegetable oil, and peppermint extract to a large bowl. Stir until a thick, uniform dough forms, then fold in the semi-sweet chocolate chips if using.

Scoop rounded tablespoons of dough, or use a 1 1/2-tablespoon cookie scoop, and arrange the portions about 2 inches apart on the prepared baking sheets.

Bake for 9–11 minutes, until the edges are set but the centers still look slightly soft. Do not wait for the centers to look completely firm, because the cookies continue setting as they cool.

Immediately sprinkle the warm cookies with crushed peppermint candies, or gently press the candy pieces into the tops. Let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack.