Whisk brown sugar, eggs and yolks in a bowl to combine. Bring cream, milk, vanilla bean and seeds, and a pinch of salt to a simmer in a saucepan over medium heat, then, stirring continuously, pour mixture over eggs and mix to combine, incorporating as little air as possible. Refrigerate to rest (at least 2 hours, or overnight).
Meanwhile, for caramel, stir sugar and 50ml water in a small saucepan over medium-high heat to dissolve sugar, then cook without stirring until very dark caramel (10-12 minutes; it should almost be to the point of burning). Divide among four 12cm-diameter ceramic ramekins and swirl each quickly to coat to the edges. Set aside to cool and set (10-15 minutes).
Preheat oven to 150C. Strain custard through a fine sieve and pour into caramel-lined moulds. Place in a roasting pan lined with a tea towel, then fill pan with warm water to halfway up sides of moulds. Cover with foil and bake until custard is just set with a slight wobble in the centre (35-45 minutes). Remove from water bath and set aside to cool completely, gently releasing the sides of the custard with a wet finger once or twice as it cools, then refrigerate to chill (2-3 hours).
To serve, run a knife around edges of custards to release, then invert onto plates and serve chilled.