Butter the panettone and cut each slice into quarters diagonally to create triangles.
Make the sweet potato juice by cutting the raw sweet potatoes into chunks and placing in a blender with a little water. Blend on pulse until very smooth and pass through a fine sieve. (Alternatively, you can use a juicer).
Whisk the egg yolks and caster sugar together in a bowl. Split the vanilla pod, if using, and add the seeds or the vanilla essence to the egg and sugar mixture. Add 350ml of the sweet potato juice and double cream and mix well.
Spread the marmalade on the bottom of a baking dish and arrange the panettone in layers, sprinkling some of the white and dark chocolate between the layers and reserve some for the top.
Spoon the sweet potato mixture over the panettone, and sprinkle with the remaining chocolate. Leave to soak for 20 minutes.
Place the baking dish in a roasting tray lined with some baking parchment (this will create a water barrier under your pudding) and the fill the tray three quarters with water. Place the oven and cook for 30-40 minutes or until the pudding begins to set.
When ready, remove from the water bath, sprinkle with the sugar and glaze under the grill on a medium heat.