Place 125 g flour, 50 g sugar, salt, 75 g fat and egg yolk in a mixing bowl and first mix with the dough hooks of the hand mixer. Then work into a smooth dough with cool hands.
Cover and let it rest in the refrigerator for about 30 minutes. In the meantime, put the remaining flour, 75 g sugar and other fat in a mixing bowl and work into a crumbly dough with your hands. Roll out the short pastry to a thickness of approx. 4 mm and cut out approx. 8 circles (each 9 cm Ø).
Place on a baking tray lined with baking paper and spread the crumbled dough on the outer rim. Leave the centre of the short pastry circle free and prick it with a fork. Bake in the preheated oven (electric: 200 °C/ gas: level 3) for 12-15 minutes.
Let it cool down on a cake rack. Meanwhile clean, wash and cut the rhubarb into pieces. Put rhubarb, 150 ml water, lemon peel, 70 ml juice and remaining sugar in a pot and bring to the boil.
Cook for 4-5 minutes at low heat. Mix the starch and remaining juice and thicken the rhubarb with it. Bring to the boil again and then let it cool down. Melt the chocolate coating in a hot water bath.
Pour into a small freezer bag, cut off a small corner and squirt strips onto the crumble thalers. Place one tablespoon of rhubarb compote in the middle of each thaler and allow to set in the fridge for about 30 minutes.
If desired, serve decorated with lemon balm. Whipped cream tastes good with it.