For the shortbread crust:
6 ounces (1 ½ sticks) cold unsalted butter, diced, plus more for greasing the baking dish
7 ounces all-purpose flour
¾ ounce cocoa powder
½ teaspoon kosher salt
3 ounces semisweet chocolate, chopped or ½ cup semisweet chocolate chips
3 ounces sugar
For the chocolate layer:
3 ounces heavy cream
4 ounces semisweet chocolate, chopped or scant 2/3 cup semisweet chocolate chips
¼ teaspoon peppermint extract
Make the shortbread crust:
Preheat the oven to 350˚F. Butter an 8-inch square baking dish and line with parchment paper. Sift together the flour, cocoa powder, and salt. Place the chocolate into a medium bowl, place the bowl over a medium pan of simmering water, and heat, stirring frequently, until melted. In a mixer fitted with a paddle attachment, beat together the butter and sugar on medium until blended. Beat in the chocolate until thoroughly combined. Add the dry ingredients and mix on low until the dough comes together, stopping the mixer once or twice to scrape down the sides of the bowl. Transfer the dough to the baking dish and press into an even layer. Bake for 30 to 35 minutes, or until firm. Let cool to room temperature.
Make the chocolate layer:
Bring the cream to a bare simmer in a small, heavy saucepan. Place the chocolate into a small bowl, add the hot cream and peppermint extract, and whisk until smooth. Pour the chocolate mixture over the shortbread and spread evenly. Let cool for a couple of hours, or until set.
Using the parchment paper, lift the bars out of the pan and transfer them to a cutting board. Cut into portions and serve.
Makes 12 bars. For a professional-looking presentation, cut the bars using a hot knife and wipe it clean between cuts. Bars keep for several days in a tightly sealed container in a cool, dry place.