Preheat oven to 350 F or 180 C. Grease your pan well with butter or oil. This recipe works for a 8 inch round and square pan but I used a Bundt Pan for a fancy and festive feel.
Whisk together the whole wheat flour, cocoa powder, baking soda and sugar.
Make 3 wells or depressions in the flour-mix: 2 small ones for the vinegar and vanilla essence and one largish one for oil.
Pour in the vinegar, vanilla essence and oil into the wells/depressions. Follow up with orange juice.
Mix gently till smooth. Be careful not to over-mix.
Pour batter into your pan, making sure it's not too full. Bake for 30 to 35 minutes till done.
Insert a toothpick in the middle to check for done-ness. If it comes clean, it is done. Cool for five minutes and turn the cake over on a dish.
Dust the cake with little powdered sugar and cocoa powder and drizzle with chocolate sauce (optional) and decorate with orange slices.