Clean and wash the rhubarb and cut it into small slices, except for 50 g. Beat the fat, sugar, lemon zest and salt with the whisk of the hand mixer until foamy. Separate the eggs. Put the egg whites aside.
Gradually stir the egg yolk into the fat mass. Mix flour (up to 1 tablespoon), hazelnuts and baking powder and stir into the fat-egg mass alternately with the milk. Turn rhubarb in the rest of the flour and fold into the dough.
1 muffin tin (12 trays) with paper cups. Spread the dough on them. Bake in the preheated oven (electric cooker: 175 ° C/ convection oven: 150 °C/ gas: level 2) for 20 minutes. Beat the egg white until stiff.
Gradually pour in 100 g icing sugar. Remove the muffins. Spread the egg whites on top and bake for another 10 minutes. Take out the muffins and let them cool down on a cake rack. Cut fine strips of rhubarb from the rest and decorate the muffins with them.