1. Heat the oven to 180C/160C fan/gas 4. Butter and line a 20 x 30cm tin with baking parchment. Melt the butter, dark chocolate, sugar and 180ml cold water in a pan over a low heat. Leave to cool, then stir in the eggs, yogurt and vanilla extract. In a large bowl, sift in the flour, baking powder, cocoa powder and bicarbonate of soda. Make a well in the centre, then pour in the chocolate mixture and mix thoroughly. Pour into the tin and bake for 45-50 mins until a skewer inserted into the middle comes out clean. Set aside to cool.
2. For the buttercream, whisk the butter for 5 mins until smooth, then sift over half the icing sugar and cocoa powder and beat for 3 mins. Add the remaining icing sugar and cocoa powder, plus the vanilla extract, then beat for 5 mins until light and fluffy (if the icing is too thick, gradually add 1 tbsp milk to soften).
3. Line a baking tray with baking parchment. Break the white chocolate into a bowl, then heat in the microwave in 30-second bursts until melted, stirring halfway. Drop spoonfuls of the melted chocolate onto the prepared sheet, then form into ‘ghosts’ using the back of a spoon. Once set, draw on spooky faces using the writing icing.
4. Roll out the fondant icing and use it to cover each finger biscuit. Stick in place with the writing icing, then trim the edges and write spooky gravestone phrases on them. Level off the top of the cooled cake with a knife and break the trimmed off bits into crumbs to make the ‘soil’.
5. Transfer the cake to a serving plate and cover it with the buttercream. Sprinkle over the cake crumbs and chocolate sprinkles, then stick in the tombstones, add the chocolate ghosts and sprinkle some Chocolately Mix Sprinkles around each one. Press the pretzel halves around the sides of the cake to make the fence.