For smaller, thicker squares, you can bake the brownies in a 9 by 13 inch pan instead, increasing the baking time by about 5 minutes. You may need less ganache for the top layer. When making the frosting, start with the smaller amount of peppermint extract, then add more, a drop or two at a time, to suit your taste.
Preheat oven to 350 degrees. Grease 10 by 18 inch pan then line with parchment or wax paper, using enough to hang over the sides slightly.
Melt in a large saucepan over low heat
3/4 cup unsalted butter
When melted, stir in, then remove from heat
4 ounces semisweet chocolate, chopped fine
4 ounces unsweetened chocolate, chopped fine
When chocolate has melted and mixture has cooled slightly, stir in:
1 cup granulated sugar
1 cup light brown sugar
1 teaspoon peppermint extract (optional)
1 teaspoon vanilla extract
1/2 teaspoon salt
Beat in a small bowl, then add and incorporate thoroughly:
4 large eggs
Stir in until completely blended:
1 cup flour
Pour batter into prepared pan and bake 20 to 25 minutes, until knife inserted in center of brownies comes out just clean. Cool completely in pan.
Mint Frosting Layer
Beat with electric mixer 2 to 3 minutes:
1 cup (2 sticks) unsalted butter, softened
4 ounces cream cheese, softened
Add and beat 2 to 3 minutes, until smooth:
4 cups confectioners' sugar, sifted if necessary
1 to 1 1/2 teaspoons peppermint extract
1/2 teaspoon vanilla extract
Milk or cream (if necessary to make frosting spreadable)
Add until desired color is reached:
Green food coloring
Dollop onto surface of cooled brownies, then spread into smooth, even layer with an offset spatula. Refrigerate while making ganache.
Scald in small saucepan over medium heat:
1 cup heavy cream
Add and let sit off heat 5 minutes:
12 ounces (2 cups) semisweet chocolate chips
Stir until smooth. Cool until just slightly warm and pour over frosting layer, then smooth with offset spatula. Refrigerate until firm, then cut and serve.
Note: If cutting out shapes with Ann Clark cookie cutters, it's easiest to use one without a handle. Press down firmly with the cutter, then slide a pancake turner under the brownie layer to lift it up. Then, holding the cutter with one hand, use your other hand to gently push the bar up and out of the cutter.