• Preheat oven to 180°C (350°F ). Grease three 20 cm (8-inch) springform pans and line with baking paper, then grease the baking paper.
• Whisk the flour, cornstarch, baking powder, baking soda and salt together in a large bowl. Set aside.
• Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium-high speed until creamy, about 2-3 minute. Scrape down the sides of the bowl with a rubber spatula as needed. Add the oil and beat well. Add in egg yolks and vanilla extract and beat on high for 1 minute.
• With a mixer on low speed, add the dry ingredients in 3 additions alternating with the buttermilk (dry-wet-dry-wet-dry).
• Beat the 4 egg whites on high speed until stiff peaks form. Using a rubber spatula, gently fold in the cake batter. Divide the batter between prepared pans.
• Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean (be carefeul to not overbake). Start checking at 20 minutes mark.
• Remove cakes from the oven and let cool in the pans for 20 minutes, then remove the springform, and leave it to cool completely. Do not remove the baking paper until ready to assemble.

Orange Curd:

• Combine the orange zest, orange juice, butter and sugar in heatproof bowl and place over a saucepan of gently simmering water. Make sure the bowl fits snugly without touching the water. Lightly stir the mixture occasionally until the butter has just started to melt.
• Meanwhile, place the egg and egg yolk in a small bowl and beat well. Add few tablespoons of warm orange juice mixture into an eggs, whisk then carefully pour the eggs mixture into the orange mixture and whisk for a minute or so until well combined. Cook the mixture for 9–10 minutes, scraping down the sides occasionally with a spatula and giving the mixture a whisk every minute. The curd is cooked when the surface is glossy and curd is thick enough to coat the back of a wooden spoon. Cool completely and put in the fridge for at least 2 hours before assembly the cake.

Chocolate Buttercream:

• Place the chocolate in a microwave-safe bowl. Heat in 30-second intervals, stirring every 30 seconds, until chocolate is melted. Set aside to cool to room temperature.
• In a small saucepan, combine flour, cocoa powder, sugar, milk and salt. Whisk to combine. Bring to a simmer over medium heat. Simmer for 1-2 minutes, whisking frequently, until thickened to pudding-like consistency.
• Remove from heat and pour into a small bowl, cover with plastic foil, but make sure foil is touching the pudding to avoid making a skin on top of it. Put it in the refrigerator to speed cooling but keep in mind that pudding needs to be room temperature because later we wil mix in a softened butter.
• Once the pudding has cooled, beat the butter bowl until smooth and fluffy and lightened in color, around 5 minutes. Add the cooled pudding (which will look like glue at this point) one tablespoon at a time, mixing well after each addition. Beat until fluffy, about two minutes, scraping down the sides of the bowl as necessary. Add the melted chocolate and vanilla and beat until smooth and fluffy, about another two minutes.

Assembly:

• Using a cake leveler or a serrated knife level the tops of your cakes if needed.
• Place one layer of the cake on cake stand or plate. Using offset spatula or piping bag fitted with round tip, spread third of chocolate buttercream and make a cream border. Fill it with orange curd, be sure not to overfill it. Repeat with the second layer of cake, chocolate buttercream and orange curd. Top with third cake layer. Spread the remining buttercream on the top of the third layer and the sides with an offset spatula. Put the cake in the fridge for about 30 minutes before you pour chocolate ganache.

Chocolate Ganache:

• Place chopped chocolate and honey, if using, in a bowl. Heat the cream in a small saucepan until it begins to gently simmer, then pour it over the chocolate. Let it sit for 2-3 minutes to gently soften the chocolate, then stir until smooth. You should get a smooth runny ganache that's easily pourable.
• Using a teaspoon, apply ganache near edges like so to create the drips. Pour some ganache on the top of the cake and spread with an offset spatula.
• Keep it in a fridge, wrapped in cling film, but take it out of it at least half an hour before serving.