Combine the mascarpone, sugar, orange zest and juice. Whip the cream until stiff, fold in carefully. Cover and chill for approx. 15 mins. Transfer the mascarpone cream to a piping bag with a smooth nozzle (approx. 1 cm in diameter), pipe onto 8 biscuits. Use the biscuits to create 4 towers, cover and chill for approx. 1 hr.
Bring the sugar and water to the boil without stirring. Reduce the heat and simmer, swirling the pan occasionally until a light brown caramel has formed. Remove the pan from the heat, stir in the butter. Pour in the cream and water, simmer and stir over a low heat until the caramel has dissolved, reduce for approx. 5 mins. Leave the caramel to cool.
Plate up the towers, drizzle the caramel on top, sprinkle with fleur de sel.