Place yeast dough plates next to each other on the work surface and allow to defrost. Cut the dough plates in half crosswise, knead well and form into balls. Place them side by side in a greased, ovenproof dish and leave to rise in a warm place for about 45 minutes, covered. Melt the butter and brush the balls with it. Bake in the preheated oven (electric: 200 °C/ gas: level 3) for about 20 minutes. Meanwhile, clean, wash and chop the rhubarb for the sauce.
Bring rhubarb, sugar, sliced vanilla pod and 100 ml water to the boil and let it boil for about 5 minutes while stirring. Mix starch with some water until smooth and stir into the rhubarb sauce. Bring to the boil again briefly. Sprinkle the beans with cocoa and icing sugar. Let the sauce cool down and serve it to the pasties. Decorate with lemon balm as desired