Mix the flour, yeast, and 150ml warm milk into a thick, smooth ferment.
Rest the ferment at room temperature for at least two hours.
Combine the ferment with batter ingredients in a mixer and beat for 10 minutes until elastic.
Grease the moulds heavily with butter.
Fill the moulds two-thirds full with batter using a piping bag or wet hands.
Prove in a warm place for 40-50 minutes until risen 1cm above the rims.
Preheat the oven to 190°C.
Boil the citrus peel, juice, sugar, and 500ml water until slightly thickened.
Remove syrup from heat, stir in 200ml rum, and allow to cool until warm.
Bake the babas for 10-15 minutes until golden brown.
Turn babas out of moulds and cool on a wire rack.
Soak the babas in the syrup for 2-3 hours until soft.
Serve in bowls topped with citrus peel, extra syrup, Chantilly cream, and rum.