Preheat oven to 350F, grease three 6" round baking pans and dust with cocoa powder. Line bottoms with parchment.
Place all dry ingredients into the bowl of a stand mixer fitted with a paddle attachment. Stir to combine.
In a medium bowl whisk all wet ingredients (pour hot water in slowly as not to cook the eggs).
Add wet ingredients to dry and mix on medium for 2-3 mins. Batter will be very thin.
Pour evenly into prepared pans. I used a kitchen scale to ensure the batter is evenly distributed.
Bake until a cake tester comes out mostly clean. A total of 30-35mins.
Cool 10 minutes in the pans then turn out onto a wire rack to cool completely.
Mint Chip Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a hot water bath on the stove and whisk constantly until the mixture is no longer grainy to the touch (approx. 3mins).
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch -- approx. 5-10mins).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth. Add peppermint extract.
Stop the mixer. Dip a toothpick into the green color gel then dip it into the frosting. Whip until smooth and evenly green. Add more color gel if desired.
Fold in chopped chocolate.
Place chopped chocolate and cream into a microwave safe bowl. Stir to combine. Microwave for 20 seconds, stir. Microwave in 10 second intervals, stirring in between, until ganache is smooth and silky. Set aside to cool completely and thicken slightly before using on cake. See notes.
Place one layer of cake on a cake stand or serving dish. Top with about 2/3 cup buttercream. Repeat with remaining layers. Frost and smooth the outside with a thin crumb coat. Chill for 20mins.
Frost the top and sides of the cake.
Using a small spoon, place dollops of ganache around the top edges of the cooled cake, allowing some to drip down. Fill in the top of the cake with more ganache and spread evenly with an offset spatula.***
Top with fresh mint if desired.
* Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen. ** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. *** You may have some ganache left over.For the ganache, I let mine set overnight (with plastic wrap placed directly on the surface). It will be too thick at this point, so I microwave it for 5-10 secs and stir it until it's the right consistency.