Preheat the oven to 350°F and prepare two 8 inch round cake pans. Spray the inside with cooking spray, then cut round pieces of parchment paper so that they fit in the bottom of each pan (do not skip the parchment paper lining!) Whisk flour, cocoa powder, sugar, salt, baking powder, baking soda together in a large bowl.
Add in the eggs, vanilla, oil, orange zest, orange juice, and milk. Pour in the hot water and mix until smooth.
Divide the batter evenly between the two cake pans and bake on the middle rack of the oven for 20-26 minutes or until a toothpick inserted in the middle of the cakes comes out clean.
Let the cakes cool in the pans, then run a butter knife around the outside of the cake to loosen it from the pan and flip upside down onto a cooling rack.
Make the buttercream. In the bowl of a standing mixer or handheld mixer, beat butter and shortening together until combined. Skip if using all butter. Beat in the cocoa powder until smooth.
Add in the orange zest, then add the icing sugar, ¼ cup at a time until desired consistency is reached. If the buttercream is too thick, add in milk 1 tablespoon at a time to thin it out to the consistency you want. If it's too thin, add in more confectioner's sugar (powdered sugar) ¼ cup at a time.
Once the cakes have cooled completely, frost them and enjoy!