Rich, fudgy dark chocolate brownie cookies get a festive peppermint finish with silky chocolate ganache and crushed peppermint candy. Bake the cookies just until set for soft centers, then chill them for an even fudgier texture.
Heat the oven to 350°F (175°C). Line a baking sheet with parchment paper if desired.
In a large bowl, use an electric mixer on medium-high speed to beat the softened butter, brown sugar, and granulated sugar for about 2 minutes, until light and fluffy.
Mix in the egg, ⅛ teaspoon peppermint extract, vanilla extract, and corn syrup on low speed until combined. Scrape down the sides of the bowl.
Add the baking soda and salt and mix briefly. Add the cocoa powder and flour, then mix on medium speed just until combined.
Scoop the dough into ¼-cup portions and roll each portion into a ball. Gently flatten each ball into a disc about ½ inch thick to help the cookies spread as they bake.
Bake at 350°F for 11–12 minutes. The centers may look slightly wet; avoid overbaking, as they will continue to set on the hot baking sheet. Let the cookies cool completely before decorating.
Place the chocolate chips in a heat-safe bowl. Heat the cream in a microwave-safe container for 30–45 seconds, until it begins to bubble, or warm it on the stovetop until steaming. Carefully pour the hot cream over the chocolate chips, press the chips beneath the cream, and let stand for 3 minutes. Stir until smooth, reheating in 10-second increments at 50% power if needed. Stir in ¼ teaspoon peppermint extract.
Spoon about 1 tablespoon of ganache onto each cooled cookie and sprinkle with crushed peppermint candy. Refrigerate for 30–60 minutes, until the ganache firms up.