Preheat the oven to 190ºC/375ºF/Gas Mark 5. Grease an eight-inch round sandwich cake tin and line the base with greaseproof paper. Sift the flour and baking powder together and set aside. Melt the chocolate in a heatproof bowl, or in a pan over a pan of simmering water.
Alternatively, melt the chocolate in a bowl in a microwave oven on HIGH for one minute. Cream the butter with the sugar until light and fluffy. Beat in the vanilla essence, then gradually add the beaten eggs, beating well after each addition.
Stir in the melted chocolate and hung yogurt evenly. Fold in the flour. Pour the batter into the prepared cake tin and bake for twenty to twenty five minutes, or until firm to the touch. Turn out onto a wire rack to cool. When completely cold, cut the cake in half horizontally.
To make the icing, melt dark chocolate as above. Stir in the powdered sugar and hung yogurt. Stir the mixture with a rubber spatula until smooth; beat until the icing begins to cool and thicken slightly.
Use about a third of the mixture to sandwich the cooled cake halves. Working quickly, spread the remaining icing over the top and decorate with chocolate curls.