Grease and line the base of an 8 inch / 20cm round cake pan.
Using a dry microwave safe bowl, melt the chocolate in the microwave until completely melted. About a minute should be fine.SEE NOTE 1.
Separate the egg yolks and egg whites and using a hand mixer, whisk the whites until soft peaks form.
Add the yolks, one by one, into the bowl of chocolate, whisking to combine.
At first it may look like the chocolate is separating or drying. Do not worry about this, because as you add the rest of the yolks, it blends well into the chocolate creating a creamy consistency. Mix in the orange zest.
Using a rubber spatula or metal spoon, gently fold ⅓ of the whipped egg whites into the yolk and chocolate orange mixture. Repeat with the remaining egg whites, adding ⅓ at a time.
Pour the batter in the greased and lined pan.
Bake for 20 to 25 minutes in a preheated oven of 320°F /160°C.
Remove from the oven and run a knife or spatula around the edges of the cake to loosen it from the pan.
Once the cake is cool to the touch, gently invert it onto a cake stand or serving plate.
Dust with powdered / icing sugar and serve with a scoop of ice cream or whipped cream if preferred.
1. Just make sure that the utensils, you are using for the chocolate is completely dry. Water and chocolate do not mix. If the chocolate comes into contact with water, it will cease, making it unusable.