Preheat the oven to 350ºF and grease three 8-inch cake pans with baking spray or butter and flour. Line the bottom with a parchment round, and wrap them with fabric soaked baking strips, if you have them.

In a large bowl whisk together the flour, cocoa powder, baking soda, baking powder, espresso powder, and salt. Set aside.

In another large bowl combine the vegetable oil, granulated sugar, brown sugar, eggs, sour cream and vanilla and whisk until smooth. With a spatula, fold the zucchini into the wet ingredients until fully incorporated.

Pour the wet ingredients into the dry and stir until almost combined (there should be a few dry streaks remaining). Add in the chocolate chips and stir in until no dry streaks remain and the chocolate is well dispersed throughout the batter.

Divide the batter evenly among the cake pans (about 3⅓ cups/810g each). Bake for 35 to 40 minutes or until the centers are springy to the touch and the cakes start to pull away from the sides of the pans. Allow the cakes to cool in the pan for 10 minutes before inverting onto a wire rack to cool completely. Remove the parchment paper once cooled.

Place the chocolate chips and ⅓ cup of the heavy cream in a small microwave safe bowl or a liquid measuring cup. Microwave in 30 second intervals, stirring between each, until the chocolate is fully melted. Set aside to cool while you make the frosting.

In the bowl of a stand mixer fitted with a paddle attachment, cream the butter and salt on medium speed until light and creamy, about 3 minutes. Sift in the cocoa powder and mix on low until combined. Scrape down the bowl.

While mixing on low, gradually add the powdered sugar, about 1 cup at a time, alternating with a small pour of the remaining ⅓ cup of heavy cream. Once all of the powdered sugar and cream is incorporated, add in the vanilla extract and melted cooled chocolate.

Increase the mixer to medium speed and beat for 2 minutes or until the buttercream is very light, fluffy, and easy to spread. Transfer 1 cup (236g) of the frosting to a piping bag fitted with an open star tip (Ateco 869.)

Place a cooled cake layer on a cake stand or serving plate and top with 1 cup of buttercream, spreading into an even layer all the way to the edge. Top with another cake layer and repeat with the buttercream. Top with the remaining cake layer, placing it top side down so the top of the cake is even. Spread the remaining buttercream all over the top and sides of the cake. I prefer to leave a thicker layer on top, and spread a thinner layer on the sides, then use an offset or bench scraper to remove most of the frosting on the sides. This gives the cake a naked cake look and a perfect balance of frosting to cake.

Finish decorating the cake by piping dollops of buttercream around the top edge of the cake. For clean slices, chill the cake for at least 30 minutes before slicing so the frosting firms up a bit and the cake is easier to slice.