Preheat the oven to 350°F(177°C). Grease the bottom and sides of two 8-inch round cake pans and line the bottom with same-size parchment paper rounds and set aside the pans.
Place the flour, cocoa powder, baking powder, baking soda and salt in a large bowl and stir with a whisk. Set aside.
Combine the orange zest and sugar in a medium bowl either rubbing with your fingers or stirring with a fork to bring the orange flavor out.
Add the eggs and stir with a whisk until combined for 20 seconds or so.
Add the melted butter, buttermilk and orange juice and stir with a whisk.
Pour the wet ingredients over the flour mixture and stir with a whisk.
Mix in the hot water.
Divide the mixture into 2 greased and parchment paper lined pans and bake for 24 minutes or until a toothpick that is inserted into the center comes out clean.
Prepare the ganache: While the cake is baking, you can make the ganache. First let it cool at room temperature and then keep it in the fridge until it becomes a spreadable consistency.
Once cakes are baked, remove them from the oven and place on a wire rack to let them completely cool in the pan. After 20 minutes gently release the cake from the sides with a small spatula so you can remove them easily from the pan once needed. As they cool at room temperature, cover the pans with parchment paper and continue to cool in the fridge at least for an hour . This way you can level them (there may be domes in the middle part of the cake so you may need to level them to make them flat) and apply the frosting easily as they set.
If you want to decorate the cake with candied orange slices, you can make the recipe while waiting for the ganache and cake to set.