Preheat the oven to 170 C / Gas 3. Grease and line two 20cm cake tins with baking parchment.
For the cake:
In a mixing bowl, beat the butter and sugar together until light and fluffy. Add the vanilla extract and then one egg at a time, beating until smooth after each addition. Add the yoghurt and beat until smooth. Sift in the flour then add in the cocoa powder and mix well. Spoon into the prepared cake tins.
Bake in the preheated oven for 25 to 30 minutes until a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool in their tins for 1 hour then transfer to a cooling rack for 2 hours to cool fully.
For the icing:
Once the cakes are cool and ready for decorating, melt the chocolate in a bowl over hot water. Add the butter and stir until melted. Remove from the heat and stir in the icing sugar and golden syrup.
Put the first cake onto a serving plate and slice a little off the top (if necessary) to make it an even base for the top layer. Spread the icing over this first layer then sandwich the second cake on top. Spread the remaining icing on top then dust with icing sugar. Slice and serve.