Preheat your oven to 350 degrees Fahrenheit. This step is crucial as it ensures that your cupcakes bake evenly. Line a 12-cup cupcake tin with paper liners and lightly spray them with non-stick spray; set aside until needed.
In a heatproof bowl, combine the canola oil and chopped semi-sweet chocolate. Melt them together in the microwave, using 30-second increments, stirring in between each until completely melted and smooth. This mixture will serve as the rich base for your cupcakes.
In a medium-sized bowl, combine the all-purpose flour, baking soda, baking powder, cocoa powder, ground cinnamon, and salt. Stir together until thoroughly combined. This dry mixture is essential for achieving that perfect cupcake texture.
In a large bowl, whisk together the eggs, egg yolk, granulated sugar, light brown sugar, and vanilla. Beat until the mixture is smooth and creamy. This combination will add sweetness and richness to your cupcakes.
Next, add the melted chocolate and oil mixture to the egg mixture, whisking until smooth. Then, incorporate the sour cream. This will ensure your cupcakes are rich and moist.
Gently fold the dry ingredients into the wet ingredients using a rubber spatula until barely combined. Be careful not to overmix, as this can lead to dense cupcakes.
Stir in the hot coffee or hot water until just combined. This step is important for enhancing the chocolate flavor while keeping the mixture light.
Finally, fold in the shredded zucchini, stirring just until everything is well combined. The zucchini adds fantastic moisture without altering the chocolatey flavor.
Fill each cupcake mold about three-quarters full with the batter. Don’t worry if you have extra batter; you can bake a second batch later!
Bake for 20 minutes, or until a toothpick inserted in the center comes out clean. Once done, let the cupcakes cool completely before frosting.
For the frosting, sift together the confectioners' sugar and cocoa powder. Using a handheld mixer or a stand mixer fitted with the paddle attachment, beat the unsalted butter on medium-high speed until creamy, about 2 minutes. This will form a fluffy base for your frosting.
Reduce the speed to low and slowly add the sifted sugar/cocoa mixture, alternating with sour cream and milk. Add in the salt and vanilla and beat on medium-high speed until everything is light and creamy, about 2 minutes.
Once cooled, frost the cupcakes generously and feel free to top them with any decorations you like. These cupcakes can be stored in an airtight container in the fridge for up to 4 days, but they’re best enjoyed at room temperature.