Preheat the oven to 350F/177C and line two, 8-inch cake pans with parchment paper and spray the sides with baking spray. Wash the zucchini and grate into a large mixing bowl; no need to peel the zucchini.
Next, prepare the cake batter. Into a large mixing bowl, add the eggs, oil, milk, sugar, extract and instant coffee. Using a hand mixer, whisk or stand mixer, mix the ingredients together for 3 to 4 minutes, until the eggs are well beaten. Melt the chocolate in the microwave or over a double boiler, then add into the cake batter and mix for 1 minute. In a separate bowl, combine the dry ingredients: flour, cocoa powder, baking soda, baking powder and salt. Sift the dry ingredients into the cake batter, then mix for about 1 minute, just until the dry ingredients are incorporated. Watch my video recipe to see how it's done!
Next, fold in the grated zucchini using a spatula. Divide the cake batter evenly between the two pans. Bake in the preheated oven for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean. Remove the cake layers from the oven and let them cool in the pans for about 15 minutes, then remove them onto a wire rack and cool completely. Once the cake layers are cooled, use a long serrated knife to split the layers in half.
Making Chocolate Buttercream:
While the cake layers are cooling, prepare the chocolate buttercream. Place the softened (not melted!) butter into a stand mixer bowl and whisk on high speed for 7 to 8 minutes, scraping down the sides of the mixing bowl often, until the butter is fluffy and white in color.
Next, add in the melted chocolate and vanilla and mix again for a minute until the chocolate is incorporated. Next, add in the cocoa powder and confectioner's sugar. Whisk on low speed first until the sugar is incorporated, then whisk on high speed for 3 to 4 minutes, until the buttercream is light and fluffy.
Assembling the Cake:
To assemble the cake, first level the top of each cake layer using a long, serrated knife, if needed. Add a generous amount of the chocolate buttercream in between each cake layer. If desired, frost the top and sides of the cake with frosting also. If desired, soak the cake layers very lightly with simple syrup or chocolate liqueur. Keep the cake refrigerated if not serving the same day. The cake will keep well in the refrigerator for up to 4 days.