For the ferment, place the flour in a large bowl. Crumble the fresh yeast into the bowl by rubbing it between your fingertips or add the instant yeast. Whisk in the milk until the ferment is thick and smooth. Set aside for at least two hours at room temperature and out of draughts. The ferment will start to bubble.
Add the ferment to the bowl of a freestanding mixer with a dough hook attached. Add all the batter ingredients and beat until the mixture is smooth and elastic (this will take about 10 minutes).
Grease the moulds heavily with butter. I find it easier to pipe the mixture into the moulds using a large piping bag fitted with a large plain nozzle, but, if you don’t want to do this, you can just moisten your hands with water then scoop out small pieces of batter with your fingers and drop them into the moulds. Either way, fill the moulds two-thirds full.
Place the moulds on baking trays and leave in a warm place for about 40-50 minutes, or until the mixture has risen about 1cm/½in above the rims of the moulds.
Preheat the oven to 190C/375F/Gas 5.
For the syrup, put the orange and lemon peel in a pan. Add the orange and lemon juice, sugar and 500ml/18fl oz water. Bring to the boil, then turn down the heat and simmer for about 5-10 minutes, or until it thickens slightly to form a syrup. Take off the heat, add the rum and allow to cool until just warm.
Pour the syrup into a large dish wide enough to hold all the babas (you may need to use more than one dish).
Bake the babas in the preheated oven for about 10-15 minutes, or until they are golden-brown and have risen up to resemble champagne corks. Carefully turn each one out of its mould and leave cool on a wire rack. (At this point, any babas that you don’t want to use immediately can be stored for up to three months in the freezer.)
Put the babas into the syrup, turning them to coat thoroughly, and leave for 2-3 hours at room temperature so that they soak up as much syrup as possible. Very gently prod them occasionally - when they feel soft, they are ready.
To serve, place a baba in each serving bowl. Scoop out some of the strips of peel from the syrup – a pair of kitchen tweezers is ideal for this – and curl on top of each baba. Spoon over a little of the syrup and serve with Chantilly cream and a little extra rum.