First, preheat oven the 180°C/ 160°C Fan/Gas Mark 4. Line a 12-hole muffin tray with the rainbow cupcake cases. 

In a bowl, pop in the margarine and sugar. Using a hand mixer or a stand mixer beat until light and fluffy. 

Add in the eggs, vanilla, flour, cocoa, and milk. Beat until combined.  

Fill the cupcakes 1/3 full. Pop in 1 Oreo into each cupcake. Split the rest of the mixture evenly into the cupcakes.  

Bake the cupcakes for 15 -20 minutes until risen and when a skewer is inserted into the centre of a cupcake it comes out clean. Leave to cool in the tin for 10 minutes and then remove from the tins and place on a wire rack to cool completely.  

In a bowl pop in the butter and beat for 5 minutes until light and fluffy.  

Add in the icing sugar, vanilla, and blue food colour. Beat until combined (If you would like your buttercream darker then add in more food colour) Add the buttercream to a piping bag fitted with a star nozzle.

Pipe a swirl of buttercream on top of the cupcakes.  

Add the cookie into the middle of the buttercream and then two eyes on top.  

There you have your cute little cookie monster cupcake! 

To make your own edible eyes you will need to get 100g Dr.Oetker White Fondant Icing and some Dr.Oetker Black Food Colour Gel. Split the fondant in half, and colour on half black with ½ tsp of black colour gel. Then roll out some little balls of the white icing, and flatten in your finger, and the same with the black. Stick the two circles together, and there you have the eyes.