Place the butter in a bowl, whisk in the sugar and salt. Add the eggs one at a time and continue to whisk until the mixture becomes lighter in colour. Stir in the yoghurt.
Mix the flour, baking powder and sodium bicarbonate, stir into the mixture. Divide the cake mixture into 2 portions. Mix the milk and vanilla into one half of the cake mixture.
Place the chocolate in a thin-sided bowl and suspend over a gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate and stir until smooth. Stir into the other half of the cake mixture along with the tangerine juice. Mix the tangerines into both halves. Using tablespoons, spoon the cake mixtures alternately into the cake mould.
Approx. 50 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, tip out onto a cooling rack and leave to cool completely.
Mix the icing sugar and tangerine juice, drizzle over the Gugelhopf.