Ingredients
Cake Ingredients  
40g room temperature butter 
125g plain flour 
1½ tsp baking powder 
140g caster sugar 
1 large egg 
120ml milk 
1 tsp vanilla extract 
10g cocoa powder  

Icing 
175 icing sugar 
60g room temperature butter 
15ml milk 
¼ tsp vanilla extract 

Method
Heat the oven to 180c. 

Put the butter, flour, baking powder and sugar either in a mixer with paddle attachment or large bowl for an electric whisk or just with a wooden spoon.  The wooden spoon works fine, it just takes a few minutes more.  When my Saturday cooking club made these none of us used the mixer.  Work the butter into the dry ingredients until it looks like fine breadcrumbs. 

Put the both the egg and the vanilla extract into the milk and mix.  Pour half of the milk mixture into the flour mixture and mix, when incorporated add the rest of the milk and mix.  Take out half of the mix.  Put the cocoa powder in one half and mix until completely incorporated, 

Line a cupcake tin with cupcake cases and put equal amounts of the vanilla mix into each of the cupcake cases.  Repeat with the chocolate mixture. Get a skewer or cocktail stick and use it to marble the two flavours.  Cook for 20-25 minutes.  Cook until the cake feels firm and a skewer comes out clean. 

If you have a fairy cake tin and fairy cases which are slightly smaller these can be used, they will just cook quicker so will only take 12-15 minutes. 

While the cakes are cooking, make the icing.  Again this can be done by machine or hand.  Sift the icing sugar.  Put the butter into the bowl - if you are doing this in a machine put all of the icing sugar in and mix on a slow speed until incorporated.  If you are doing by hand put a third of the icing sugar in and carefully mix through before adding another third, completely mix in and then add the last third. 

Add the vanilla extract to the milk and add to the icing bit by bit.  Once all is added, beat until light and fluffy.  If you want to you can split the mixture and add 2 tbsp of cocoa powder and thoroughly mix so you have marbled icing. 

When the cupcakes are cooked, get them out and leave them to cool completely.

To Decorate 
Smarties, jelly tots, mini pretzels, mini flakes, silver balls, hundreds and thousands, glitter, gold stars etc etc etc! 

Spoon or ice the top of each cake and adorn however you like.