Scoop the solid cream from the top of one can of coconut milk and melt in a saucepan over a medium high heat until it 'cracks,' until the moisture evaporates and nearly pure fat stays behind. Add the red curry paste and sauté for 5 to 10 minutes, until it is very fragrant. Add the sugar, stir for a moment, and then add the fish sauce and stir to combine. Add the beef, the remaining coconut milk, bean sprouts, carrot, lime leaves and lime juice simmer just long enough to cook the beef through. At the last minute roughly chop the coriander, stems and all, and add to the curry.