Bring lemonade to the boil in a saucepan and boil down to about 120 ml in about 10 minutes. Let cool down. Cream butter and 250 g sugar with the whisk of the hand mixer. Stir in eggs one after the other.
Mix flour and baking powder. Add 100 ml lemonade and flour mixture and stir to a smooth dough.
Pour the dough into a greased, flour-spread pudding mould or narrow, tall ring cake mould (approx. 2 litres capacity). Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 1 hour.
Take it out, let it rest for about 10 minutes, then turn out of the mould.
Mix icing sugar with the remaining lemonade and decorate the cake with it. Let the cake cool down. Whip the cream until stiff and add 2 tbsp. sugar. Fold in sour cream. Turn the cake over and spread the cream on the cake.
Sprinkle with coloured sugar.