Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a bowl, whisk together the flour, baking powder, baking soda, and salt.

In a large bowl, beat the butter and sugar together until light and fluffy. Add the eggs one at a time, beating well after each addition. Mix in the orange zest and vanilla extract.

Mix the orange juice and milk in a separate small bowl.

Gradually add the dry ingredients to the butter mixture, alternating with the orange juice mixture, starting and ending with the dry ingredients. Mix until just combined.

Divide the batter evenly between the prepared pans. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then turn them out onto a wire rack to cool completely.

In a large bowl, beat the butter until creamy for the frosting. Gradually add powdered sugar and cocoa powder, alternating with heavy cream, beating until well combined. Stir in vanilla extract.

Place one cake layer on a serving plate. Spread a thick layer of chocolate frosting over the top.

Place the second cake layer on top, then frost the top and sides of the cake with the remaining chocolate frosting.

Decorate with chocolate shavings.