Generously grease a 1.2-pint pudding basin with butter. Cut a small circle of parchment paper to fit the bottom of the pudding basin, and place it in the bottom. The butter should help it hold in place. Place an upturned plate at the bottom of a very large soup pan that is taller than the pudding basin. Fill with water half way and bring slowly to a boil.

Cut the orange down the middle and cut 2 thin slices on each side: you will have 4 thin orange slices. Set aside. Zest the remaining ends of the orange, then cut the peel away, and chop up the flesh of the orange. Place the zest and flesh is a large bowl.

Add the sugar, melted butter, milk, vanilla, and eggs. Beat well. Add the flour, salt, and cocoa and mix very well until smooth. Spoon the mixture into the pudding basin, smoothing to the edges.

Position the 4 thin slices of orange around the base, pushing against the sides of the basin. Spoon the mixture into the center of the pudding basin, smoothing to the edges and top.

Place a 30 cm long (1 foot)  piece of parchment paper on top of a sheet of aluminum foil of the same side. Grease the parchment with butter in the center. Pleat the parchment and aluminum papers in the middle and fold over about 3 cm (1 inch). Place both sheets over the pudding and fold edges around the basin. Tie a string tightly twice around the lip of the basin. Leaving 2 long ends, tuck under the string across from the knot and tie to form a handle. Trim the excess parchment and foil to about 5 cm (2 inches), and tuck the foil around the parchment to seal.

Gently place the basin into the rolling water, making sure the water is about 2/3 the way up the sides of the basin. Place the lid of the pan on top, adjust heat to a rolling simmer, and steam for 2 to 3 hours. Remove the basin from the water and leave on the countertop while you prepare the sauce.

Add the butter, sugar, cocoa, vanilla, and milk to a small saucepan. Stir over a medium-high heat. Bring to a bubble for 1 min until the sauce is glossy.

Unwrap the sheets from the basin and spoon evenly 1/4 cup on the sauce on the base of the pudding. Place your serving plate over the basin and invert to unmold. Spoon a little bit of the sauce on top of the pudding. Cut slices of the pudding and each guest can add extra sauce to their piece.