Method

• 01
  Preheat the oven to 350 degrees Fahrenheit. Line two 9-inch cake pans with parchment paper and spray them well with nonstick cooking spray.
• 02
  In a small bowl, whisk or sift together the cocoa powder, flour, salt, and baking soda. In the bowl of a large stand mixer, combine the butter and the granulated sugar. Beat them together on medium-high speed for 4-5 minutes, until they’re light, fluffy, and creamy. Add the eggs one at a time, and beat until they’re fully incorporated.
• 03
  Combine the water and the milk. With the mixer running on low speed, add about a third of the sifted dry ingredients to the bowl. When they are almost completely mixed in, pour in half of the water-milk mixture, and let it mix together
• 04
  Add half of the remaining drys, then when those are mixed, add the remaining wet ingredients. Finally, finish off by adding the last of the drys. Stop when the streaks of flour are almost completely gone. Add the miniature chocolate chips and chopped mints, and stir everything together by hand, scraping the bottom and sides of the bowl to make sure the batter is well-mixed.
• 05
  Divide the cake batter evenly between the pans and bake at 350℉ for 22-25 minutes, until the tops spring back lightly when touched and a toothpick inserted into the center comes our clean. Let cool on a wire rack for 15 minutes, then invert from the pans and let cool completely.
• 06
  Using a box grater, grate the dark chocolate until it is in small chunks or shards. Set aside for now.
• 07
  Combine the egg whites and the granulated sugar in the bowl of a large stand mixer, and whisk them together. Choose a small saucepan that lets you fit the base of the stand mixer snugly into the top of the saucepan—this is your makeshift hot water bath. (Alternately, you can use a different bowl or an actual bain marie and then transfer the mixture to a mixing bowl once it’s heated.)
• 08
  Add an inch of water to the bottom of the saucepan, and bring the water to a simmer. Place the mixing bowl on top of the saucepan, making sure that the bottom isn’t in contact with the water, and heat the egg white mixture. Whisk frequently so that the egg whites don’t cook. Continue to heat the whites until they are hot to the touch, and when you rub a bit between your fingers, you don’t feel any grittiness from the sugar.
• 09
  Once the whites are hot, transfer the mixing bowl to your mixer and fit it with a whisk attachment. Beat the whites on high speed until they are no longer warm to the touch—feel the outside of the bowl, and make sure that it is around room temperature.
• 10
  Reduce the speed to medium-low and add the room temperature butter in small chunks, a tablespoon at a time, making sure to wait in between additions. It may separate or look a little gloopy at this point—fear not. Once all of the butter is added, increase the speed again and whip until it comes together and is light and fluffy.
• 11
  Add the mint extract and a few drops of green food coloring, until you get a nice light mint green. (A drop or two of yellow might help as well, depending on your food coloring.
• 12
  Stir in the shredded chocolate by hand. The buttercream can be made in advance and kept at room temperature if you’re going to use it the same day, or refrigerated. If it’s been chilled, let it sit at room temperature until it softens, then re-whip it to get the fluffy texture back before you use it.
• 13
  Place the chocolate in a medium bowl. Pour the cream into a small saucepan, and heat it over medium-high heat until it simmers and is just under a boil. Pour the hot cream over the chopped chocolate and let it sit for one minute to soften the chocolate.
• 14
  Whisk the cream and chocolate together until the chocolate melts and the ganache is smooth and shiny. Set aside until it is warm and still fluid, but not hot. This can be made in advance and re-warmed in the microwave before assembling the cake. You will probably have some ganache left over after decorating the cake, and you can either serve it with the cake, or chill it and roll it into balls to make truffles.
• 15
  Chill the layers—I like to wrap them in plastic wrap and place them in the freezer for about 30 minutes, until they are cold but not rock-hard. When they are slightly frozen they are sturdier and easier to cut and move without crumbling.
• 16
  Use a long serrated knife to cut the cakes in half. Place one half on a cardboard cake round, and spoon about a cup of frosting on top of it. Use an offset spatula to spread the frosting evenly across the top, going all the way to the sides of the cake.
• 17
  Place another cake round on top of the frosted one, and make sure the sides line up evenly. Add more frosting to this top round, and continue to layer the cake and spread frosting. Once all the layers are stacked, use a spatula to spread more frosting on the top and sides of the cake. This frosting won’t ever get super-smooth, due to the chocolate chip bits, so you don’t have to worry about getting the sides perfectly even.
• 18
  Spoon the remaining frosting into a piping bag fitted with a large star tip. Pipe rosettes around the border of the cake, making sure they touch each other.
• 19
  Warm up the ganache, if necessary, until it is pourable and smooth. Pour a thin layer into the center of the cake and spread it to the inside edges of the rosettes. Top the cake with more Andes mints if desired.
• 20
  Chill it to set the ganache, for about an hour, then bring it back to room temperature so that the frosting is soft and supple. For the best taste and texture, serve this cake at room temperature.