Preheat oven to 350°F. Spray 3 (8") cake pans with non-stick cooking spray. Set aside.
In a medium, microwave safe bowl, melt the butter and chocolate chips on High for 2 minutes, stirring every 30 seconds. Set aside.
Place the eggs and sugar in a large bowl. With an electric mixer, beat at high speed for 5 minutes. Mixture should be pale yellow and doubled in volume. Add vanilla and salt. Sift in the cocoa. Beat on medium speed until just combined. Stir in the melted chocolate and butter. Pour into the prepared pans. Bake for 20-25 minutes or until a toothpick comes out with some crumbs but no longer wet.
For the Buttercream
Beat softened butter on medium speed with an electric or stand mixer for 3 minutes. Add powdered sugar, cream, and mint extract with the mixer running on low. Increase to medium high speed and beat for 3 minutes. Add more powdered sugar if frosting is too thin or more cream if mixture is too thick. Once it's the desired consistency, add the food coloring and mix until completely combined.
For the Ganache - (Note: make ganache after you take the cake out of the refrigerator)
Melt butter and chocolate chips in the microwave at 50% power, stirring ever thirty seconds until it's completely melted.
To assemble the Cake
Frost each layer of cake with mint buttercream. Reserve about 1 1/2 cups of buttercream for the top decoration. Then frost the cake with the rest of the buttercream. Place in the refrigerator for 1-2 hours. After the cake is chilled, pour ganache over the top, letting it fall over the edges and drip down the sides. To decorate the top, place remaining buttercream in a large pastry bag fitted with a 1M pastry tip. Pipe 6-7 swirls on top of cake. Add Andes mints.