Preheat the oven to 350°F (180°C) and spray two 9-inch cake pans with non-stick spray and line the bottoms with circles of parchment paper.
Sift all purpose flour and cocoa powder into a mixing bowl and add sugar, baking soda, baking powder, salt and instant espresso powder. Mix together.
Prepare your homemade buttermilk by adding 1 tablespoon of lemon juice to a measuring jug and then adding room temperature milk up to the 1 cup (240ml) line. Let it sit for a minute to curdle.
Add the buttermilk, canola oil, eggs, orange extract and orange zest to the dry ingredients and stir it into a thick batter.
Now add in the hot water and use a hand whisk to whisk it into the batter. The batter will be thin.
Divide the batter evenly between the two prepared 9-inch cake pans.
Place into the oven and bake for 30 minutes or until a toothpick inserted into the center of one of the cakes comes out clean.
Let the cakes cool in the pans for a few minutes before transferring them to a wire cooling rack to cool completely before frosting.
Prepare your frosting by adding softened butter to the bowl of your stand mixer. Starting at slow speed, gradually increase speed until the butter is smooth and slightly pale in color.
Add powdered sugar, cocoa powder, orange extract and milk. Starting at slow speed, gradually increase speed and let it beat for a few minutes until it's very fluffy and smooth.
When the cakes have cooled completely, frost them with the chocolate orange buttercream frosting.
Decorate with chocolate shavings and place orange slices around the cake (optional).