Peppermint Chocolate Cookies

1-1/2 sticks butter, softened
1-1/2 cups powdered sugar
1 egg
1-1/2 cups flour
1/2 cup cocoa powder
1 pinch salt
7 ounces dark chocolate, finely chopped
1/2 teaspoon vegetable oil
2/3 cup finely crushed peppermint candy canes or candies

Using an electric mixer beat the butter until soft. With the mixer on low, gradually add the powdered sugar until creamy, then beat in the egg. In a separate bowl, whisk together the flour, cocoa and salt; add to the mixer about one-third at a time, beating at low speed until smooth.

Shape the dough into 12-inch-long cylinder. Wrap tightly in parchment or waxed paper and roll into a smooth, even log. Twist the ends to seal and refrigerate for 30 minutes; reshape the cylinder and refrigerate for 30 minutes more.

Preheat the oven to 350°; line cookie sheets with silicone baking mats. Unwrap the dough; using a sharp knife, slice into 1/3-inch-thick rounds. Place about 1 inch apart on the cookie sheets; bake until firm, 18 to 20 minutes. Let cool completely.

In a bowl, combine two-thirds of the chopped chocolate with the oil; microwave for 30 seconds, then stir until smooth. (If needed, heat for additional 10-second intervals.) Stir in the remaining chopped chocolate until smooth. Spread a scant teaspoon of melted chocolate on top of each cookie. Sprinkle the peppermints on top and let stand until set, 2 to 3 hours.