Cook and simmer the frozen rhubarb with the sugar until the rhubarb is soft and has reduced a little.For the topping, mix the sugar with the flour and salt. Work in the butter and vanilla with your hands, to make a dough. Crumble the mixture into small pieces on a cookie sheet lined with parchment paper. Bake at 425°F (220°C) until golden and crispy.For the cream filling, mix the confectioners’ sugar into the mascapone, and then fold in the yogurt. Whip the whipping cream to a stiff peak and fold into the mascarpone mixture. In a pretty glass bowl, place a layer of rhubarb, then the cream, and then the crumble on top. Repeat one more time and finish the top with the remaining crumble. Sprinkle the top with confectioners’ sugar and serve warm or at room temperature.