Preheat the oven to 180 C / Gas 4. Grease a round 20cm cake tin or large 900g loaf tin.
For the cake:
Melt butter and chocolate together in a double boiler then remove from the heat and cool for 12 to 15 minutes. Whisk in the yoghurt and vanilla, then finally, the eggs.
Mix the dry ingredients together in a bowl then add the chocolate mixture and mix well. Pour into the prepared cake tin.
Bake in the preheated oven for 40 to 45 minutes or until a skewer comes out clean. Remove from the oven and let the cake stand in the tin for approximately 15 minutes before turning out onto a wire rack to cool completely, about 90 minutes.
For the ganache:
Heat up the cream in a saucepan over a medium heat and then add half of the chocolate (broken into small pieces) and finally the butter, then stir until fully melted. Remove from the heat and allow to cool and stiffen slightly to a spreadable consistency.
Coat the cake with the ganache then grate the remaining chocolate and sprinkle over the top for decoration. Slice and serve.