EXCLUSIVE : Here’s a fun one fitting for the Cannes market. Mckenna Grace ( Regretting You ), Paul Walter Hauser ( The Naked Gun ) and Louis Partridge ( Enola Holmes ) have been set to star in revisionist rom-com The Official Mistress about the final days of the French monarchy. Matt Brown ( The Man Who Knew Infinity ) is due to direct from his own script about King Louis XVI (Walter Hauser) – the Last King of France – and his pursuit of Comtesse Madeleine de Vascone (Grace) as La Maîtresse-en-Titre aka the Official Mistress in order to quell suggestions of impotence by country and court… and the humble, lowly cook Rene Rennault (Partridge) who risks treason to become the King’s Taster in an attempt to save his first love, Madeleine. The producers are currently at work casting the fourth key role of Marie Antoinette, alongside the wider ensemble, including the roles of De Conte, D’Artois, Jacques and Luc.…