Preheat the oven to 350°F (177°C). Lightly grease two or three 8-inch or 9-inch round cake pans and line the bottoms with parchment paper for easy removal.

Shred the zucchini using the standard side of a box grater. Measure out 3 cups (roughly 360g), then spread the shredded zucchini onto a few layers of paper towels. Gently blot to remove excess moisture, but do not squeeze dry.

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

In a separate large bowl, combine the melted butter, oil, granulated sugar, brown sugar, eggs, and vanilla extract. Mix until smooth and well combined.

Gradually add half of the dry ingredients to the wet mixture and mix until mostly combined. Add half of the buttermilk and stir. Repeat with the remaining dry ingredients, then the remaining buttermilk and the hot tap water. Mix until the batter is smooth, but do not overmix.

Gently stir the blotted shredded zucchini into the batter until evenly distributed.

Divide the batter evenly between the prepared pans. If using 2 pans, add about 3¼ cups of batter per pan. If using 3 pans, add a little over 2 cups per pan.

Bake 2 pans for 30 to 40 minutes, or 3 pans for 25 to 35 minutes, or until a toothpick inserted in the center comes out with moist crumbs.

Allow the cakes to cool in the pans for 20-30 minutes, then run a knife around the edges and turn them out onto wire racks to cool completely before frosting.

In a mixing bowl, beat the butter on high for 3 minutes.

Sift in the cocoa powder and 3 cups of powdered sugar. I like to sift these in because it helps break up clumps, creating a light and fluffy frosting. Beat the frosting on low speed until combined and thick.

Mix in 3 tbsp of heavy cream, vanilla extract and salt.

Add in the additional powdered sugar (1-2 cups), mix on low. If the frosting is too thick, beat in an additional tablespoon of heavy cream.

If needed, level the cooled cakes using a cake levered or serrated knife. Spread a small amount of chocolate buttercream on the bottom of your cake plate / stand, this acts as “glue” to prevent the cake from moving. Place one cake layer over the frosting. Measure 3/4 to 1 cup of frosting and evenly spread that on top of the first layer. Top with the second cake layer and spread the remaining frosting around the sides and top of the cake. I use about 1 cup of frosting on the top of the cake.

Slice and serve. Cover leftover cake tightly with plastic wrap and store on the counter for up to 3 days.