Clean, wash, cut the rhubarb into pieces and drain. Cut off a thin slice of the chocolate with a peeler for decoration. Chop the rest of the chocolate.
Mix flour, baking powder, salt, sugar and chocolate in a bowl. Whisk eggs and milk. Add egg mixture and oil to the flour mixture. Using the whisk of the hand mixer, whisk briefly to a smooth dough. Carefully fold in the rhubarb.
Place paper baking cups in the 12 troughs of a muffin tin. Spread the dough with a tablespoon. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: stage 2) for 25-35 minutes. Leave to cool in the tin for about 10 minutes. Remove the muffins from the tin and let them cool down on a cake rack. Decorate cooled down muffins with chocolate shavings.