Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, and line the bottoms with parchment paper for easy removal.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk until well blended.
Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed for 2 minutes until smooth.
Carefully stir in the boiling water until the batter is well combined. The batter will be thin; this is normal.
Fold in the orange zest and orange juice until evenly distributed.
Divide the batter evenly between the prepared cake pans.
Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then remove from the pans and transfer to a cooling rack to cool completely.
Once cooled, frost with your favorite chocolate frosting, or a chocolate-orange frosting for an extra burst of flavor.