Preheat your oven to 350°F. Cut out three 9-inch round segments of parchment paper to line your cake pans. Spray each pan generously with nonstick baking spray, then place the parchment paper cutouts in the bottom of the pans and spray again. Make sure every bit of the pan and paper are sprayed to prevent sticking. Set the pans aside.

In a large bowl, using a stand mixer with the paddle attachment or a handheld electric mixer, combine both sugars, flour, cocoa powder, baking soda, baking powder, salt, and cinnamon. Mix on low speed until the dry ingredients are thoroughly combined. Break up any large clumps with your hands if needed.

In a separate bowl, combine the eggs, egg yolks, zucchini, Greek yogurt, oil, and vanilla extract. Mix until completely combined.

Pour the wet mixture into the dry ingredients and beat on low until just incorporated. Pour in the hot water and continue mixing until completely combined, about 1 minute. The batter will be thin.

Divide the batter evenly among the prepared pans. Bake in the preheated oven for 30 minutes, or until a wooden toothpick or cake tester inserted in the center comes out clean or with just a few moist crumbs attached.

Cool the cakes for 10 minutes in the pans before removing them and transferring them to a cooling rack. Cool the cakes completely before frosting. If you don't have enough room to bake all 3 cakes on the same rack in an even line, bake 2 at a time.

In a stand mixer with the paddle attachment or using a handheld electric beater, cream the softened butter on medium speed until completely smooth, about 3 minutes.

Turn off the mixer and sift the confectioners' sugar into the mixing bowl. Turn the mixer to the lowest speed and mix until the sugar is absorbed by the butter, about 2 minutes.

Increase the mixer speed to medium. Add the melted chocolate, vanilla extract, salt, and heavy cream. Beat for 2 minutes, then increase the speed to medium-high and continue beating for another minute. If your frosting appears too thin, add a little more confectioners' sugar. If it appears too thick, add additional heavy cream, 1 teaspoon at a time.

Using a serrated knife, carefully trim the raised top of each cake to make an even, level surface.

Transfer one cake layer to a large plate or cake stand. Spread a thick layer of frosting on top, then top with another cake layer and repeat, adding another thick layer of frosting.

Top with the final cake layer and place the cake in the fridge to set for 30 minutes. Once set, finish frosting the top and sides of the cake.

Slice and serve, or keep refrigerated for up to a week.