For the cupcakes:
1) Position a rack in the middle of the oven and preheat to 150C/Gas 2. Set the liners in the muffin cups.
2) Put the chocolate and prune juice in a microwave-safe bowl. Heat in the microwave on low power, stirring occasionally, until completely melted.
3) Whisk the flour, sugar, cocoa powder, bicarbonate of soda, baking powder and salt in a large bowl. In another bowl, beat the egg and yolk with an electric hand mixer at high speed until slightly thick and lemon coloured, about 3 mins. Slowly add the buttermilk, oil, vanilla and melted chocolate mixture to the eggs, beating until thoroughly combined. Add the flour mixture and beat until just combined.
4) Divide the batter among the muffin tins, filling them halfway. Bake until a toothpick inserted into the centre of the cupcakes comes out clean and the tops spring back when pressed gently, about 25 mins. Cool the cupcakes in the tin on a rack for 10 mins then remove from the tin and cool completely.
For the icing:
1) Beat the butter, peanut butter and cream cheese in a large bowl with an electric mixer until light and fluffy. Gradually add the icing sugar and milk, mixing until the icing is smooth. Add food colouring as desired. Refrigerate the icing until firm.
For decorating the cakes:
1) For the witches hat, score a circle in the middle of a cupcake. Cut a deep, cone-like shape with a knife held at an angle. Rotate the cupcake to complete the circle and remove the centre. Repeat with all of the cupcakes.
2) Generously scoop or pipe enough icing into the centre of each cupcake to fill the hole and to rise above the cake to make a face. Top with a chocolate wafer biscuit. Put a dot of icing in the middle of the wafer and invert the cone-shaped piece of cake on top to make a witches hat.
3) Use skinny liquorice, toasted coconut or cereal flakes for hair, and sweets for eyes and/or nose.
4) Refrigerate for up to 30 mins before serving.