Peel the orange so that the white skin is completely removed. Remove the fillets with a sharp knife between the parting skins and cut into pieces. Press the juice out of the parting skins
Mix flour and baking powder. Mix egg, butter, sugar, salt and vanillin sugar. Mix in buttermilk and orange juice alternating with the flour mixture. Fold in orange fillet pieces
Line the hollows of a muffin tray with paper cups and spread the dough in them. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: stage 2) for 20-25 minutes. Remove the muffins and let them cool down
For the icing, mix icing sugar with egg white and lemon juice. Depending on the consistency of the icing, add some more icing sugar or lemon juice. Spread the icing on the muffins. Chop the pistachios and sprinkle them on the moist icing, allow to dry