Cream the softened butter, granulated sugar, and brown sugar until smooth.
Add the egg, peppermint extract, and vanilla extract and mix until creamy.
Add the flour, cocoa powder, baking soda, and salt and mix until just combined.
Fold in the dark chocolate chunks and crushed peppermint candies.
Chill the dough for 30 minutes.
Preheat the oven to 350°F and line a baking sheet with parchment.
Scoop the dough into balls and place them evenly on the baking sheet.
Bake for 9–11 minutes, until the edges are set and the centers look soft.
Immediately sprinkle flaky sea salt over the warm cookies.
Cool on a rack before storing or serving.