Preparation

Make the chocolate base

• Position a rack in the lower third of the oven and heat to 350°F. Butter a 9×13-inch pan, then line with parchment so that the parchment hangs over the short ends of the pan by a couple of inches.
• In a medium saucepan over medium-low heat, melt the chocolate and butter, stirring until smooth. Remove from the heat, and add the cocoa powder, stirring until smooth. Set aside to cool slightly. In a small bowl, whisk the flour, salt, and baking powder.
• In a large bowl, whisk the eggs, oil, both sugars, and vanilla. Add the cooled chocolate mixture
  and whisk until smooth. Add the flour mixture and stir until just combined. Pour the batter into the prepared pan and bake until the sides of the brownie have set, the top is beginning to crack and look glossy, and a wooden skewer or toothpick inserted in the center comes out with a few crumbs, 22 to 27 minutes. cool to room temperature on a wire rack, then refrigerate while you prepare the buttercream.

Make the buttercream

• In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth. Add the crème de menthe, the extracts, and the salt. Beat on medium speed to combine. Turn the mixer to low and slowly add the confectioners’ sugar, mixing until combined. Beat on medium-high speed until fluffy, scraping down the bowl as needed, 5 to 7 minutes.

Make the ganache

• Place the chocolate in a small bowl. Heat the cream in a small saucepan until just about to boil. Pour the cream over the chocolate, and let sit for 5 minutes. Stir until smooth. Stir in the vanilla and salt. Cool to room temperature before using.

Assemble the bars

• Spread the buttercream evenly on top of the chilled brownie. Return the pan to the refrigerator
  and chill for one hour. Pour the cooled ganache over the chilled buttercream, and spread in an even layer. Sprinkle evenly with the crushed candy. Refrigerate the brownie for at least one hour.
• Remove the pan from the refrigerator and let sit for 10 minutes, allowing the ganache to soften slightly, before cutting into squares.