Preheat a barbecue for medium heat and lightly oil the grate.
Barbecue the chicken breast until the meat is no longer pink inside and shows well-defined grill marks; 5 to 8 minutes per side. Cut the chicken breast in half and set aside.
Bring the water, unsalted butter and salt to a boil in a saucepan over high heat. Remove from the heat and stir in the couscous. Cover and let stand for 10 minutes then fluff with a fork.
Melt the salted butter in a frypan over medium heat and gently stir in the capers and dates. Stir a few times to heat the capers and dates then stir in the mascarpone cheese and cream, stirring until the cheese and cream have melted together. Allow the mixture to simmer for about 3 minutes to reduce slightly then season to taste with salt and pepper.
Place the couscous on a serving platter and sprinkle with a diced date. Arrange the 2 pieces of chicken breast onto the couscous and cover with the mascarpone sauce. Sprinkle the dish with diced cucumber and tomato then drizzle lemon juice over the top to serve.