Preheat the oven to 350 F. Grease and flour two 9" cake pans.
In a large bowl add the flour, sugar, cocoa powder, baking powder, baking soda and salt. Mix well.
Add the eggs, milk, vegetable oil, and orange juice to the bowl and beat with a hand mixer for 2 minutes.
Stir in the orange zest, orange oil and boiling water.
Pour the cake batter evenly between the two cake pans. Bake for 30 minutes or until a toothpick inserted into the centre of the cake comes out clean.
Remove from the oven and let rest on a cooling rack for 10 minutes. Carefully remove the cakes from the pan and let them continue to cool on a cooling rack until completely cool. You may wish to line the cooling rack with parchment paper to keep the cakes from sticking.
For The Buttercream
In a large bowl beat the butter with a hand mixer until it is smooth and fluffy.
Add in the cocoa powder, icing sugar, milk and orange oil and continue to beat until it is smooth and creamy and all of the dry ingredients are combined. If the buttercream is too thick and you would like a thinner icing, add 1 tbsp of milk at a time to get the desired consistency.
Assembly
Place one of the cakes on a serving dish or cake stand. Ice the top with an even layer with about a 1/2"-3/4" of icing making sure to ice right to the edges.
Add the other on top of the iced layer.
Using an icing knife add a thin layer of icing to the outside of the cake to seal in any loose crumbs. Then add a thicker layer of icing all around the cake and the top making sure to fill in the seam of the icing layer and the bottom edge of the cake so that edge of the cake looks uniform all the way around.
Decorate with orange slices, chocolates, cranberries or fresh holiday garland if you wish to make it look fancy.
Notes
Substitutes:Orange Oil can be substituted for 4X the amount of orange extract. Adjust to taste. Or you can use an orange liquor like Cointreau.Storage:Store the cake covered for up to a week on the counter or freeze for up to 5 months. To freeze I recommend freezing the entire cake first before wrapping or by cutting individual slices. Place the slices on a parchment-lined baking sheet and freeze flat. Once the pieces are frozen then wrap tightly with two layers of plastic wrap.To thaw, unwrap and let thaw at room temp on a plate for about an hour.