Prick your orange 4-5 times with a fork at equal spacings, then place it into your large saucepan and fill with water so that the orange floats on top and doesn't touch the base. You need to add enough water to allow for some to evaporate during the simmering process.
Bring to a rolling boil and simmer for 40 minutes.
Remove your orange and leave until cool enough to handle, then roughly chop the whole orange up and blend with your hand blender (or food processor) until it has a pulp like consistency.
Leave to completely cool.
Preheat your oven to 150°C Fan/170°C/Gas Mark 3/325°F.
Grease your baking tin with some softened butter and line the base with greaseproof (parchment) paper.
Melt your chocolate either in your microwave in short bursts of 10 seconds, stirring in-between, or in a heatproof bowl over a saucepan of simmering water, making sure the bowl does not touch the water. Leave to cool.
Sift your flour, baking powder and cocoa together and set aside.
In a stand mixer with the paddle attachment fitted or with a hand mixer with the beaters fitted, beat your softened butter and sugars until they form a soft, paste-like and airy consistency, this can take several minutes on a medium setting.
On a low setting, beat in 1 egg at a time along with a heaped dessertspoonful of your sifted flour mixture. Make sure each egg is mixed in completely before adding another.
Beat in the rest of your flour mixture until combined.
Add your cooled orange pulp, cooled chocolate and your oil and beat on a low setting until combined.
Scrape your bowl down with your spatula to ensure all mixture from the sides and base are mixed in.
Beat on medium-high for 10-15 seconds.
Pour your cake batter into your prepared tin and bake for 50-55 minutes, or until a skewer inserted into the centre of the cake comes out clean.
Leave to cool in the tin for 10 minutes then invert onto a cooling rack to continue to cool fully.