Wash the limes, grate dry and thinly grate the peel. Squeeze 1/2 lime. Cream fat, vanilla sugar, half of the lime peel, sugar and 100 g brown sugar with the whisk of the hand mixer. Stir in eggs one after the other. Mix flour and baking powder and stir in alternately with buttermilk. Pour the dough into a greased, flour-spread ring cake mould (2 litres capacity). Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 50 minutes.
Remove the cake from the oven. After approx. 10 minutes, carefully remove from the tin and allow to cool on a cake rack. Sift icing sugar into a bowl. Add egg white and 1 tablespoon lime juice and mix to a thick glaze. Pour the icing over the cold cake and let it dry for 30-45 minutes. In the meantime, finely mix the remaining lime zest and 1 tablespoon of brown sugar in the universal chopper, then leave to dry on a plate for about 30 minutes. Sprinkle the cake with lime sugar before serving