Spread half of the sugar inside the rectangles, top with the knobs of butter. Pack the rhubarb closely together on top of the sugar, sprinkle with the vanilla sugar, lay loosely over the rhubarb.
Approx. 15 mins. in the lower half of an oven preheated to 220°C. Remove from the oven, leave to stand briefly, slide both onto a cooling rack. Remove the baking paper, allow to cool slightly, top with the micro greens and serve lukewarm.