For the Cake:
Prepare the Oven: Preheat your oven to 350°F (175°C). Grease and line a 9-inch round cake pan.
Cream Butter and Sugars: Beat butter, brown sugar, and granulated sugar together until fluffy and pale.
Add Wet Ingredients: Mix in the eggs one at a time, followed by orange zest and juice.
Combine Dry Ingredients: In a separate bowl, sift together the flour, cocoa powder, baking powder, and baking soda.
Mix Dry and Wet: Gradually add the dry ingredients to the wet mixture, alternating with milk and vinegar. Mix until just combined.
Bake: Pour the batter into the prepared pan and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Cool: Let the cake cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
For the Frosting:
Mix Ingredients: Beat powdered sugar, cocoa powder, butter, and orange juice together until smooth and fluffy.
Add Chocolate (Optional): Stir in melted semisweet chocolate for an extra rich flavor.
Decorate the Cake:
Spread the frosting evenly over the cooled cake.
Garnish with candied orange slices or chocolate shavings if desired.