Rhubarb and Raspberry Mousse Tartlets

End your meal on a sweet note with these treats that require lots of steps, precise tools and meticulous work; this will all be rewarded upon first bite. You can make the sablé cookies, the rhubarb-raspberry purée as well as the mousse in advance. The latter transforms into pretty rings thanks to the use of silicone moulds. All that's left is to assemble each element. You no longer need to run to that fancy bakery down the street.