Preheat the oven to 170°C, fan 150°C, gas 3. Line a 12-hole cupcake tin with paper cases.
Beat together the flour, sugar, butter, eggs and cocoa powder, using a handheld electric whisk. Spoon the mixture into the cases and bake for 15-18 minutes, until risen and cooked – a skewer inserted into the middle will come out clean. Let cool.
To make the icing, beat together the butter and icing sugar until smooth. Add a few drops of the green food colouring and beat until you have an even colour.
Spoon the icing into the piping bag and pipe a swirl on the top of each cake. Add 1 teaspoon of the jam to the top of each.
To make the eyes, top the chocolate buttons with the chocolate chips, securing with writing icing. Place carefully on top of the jam.
Cook's tip: if you're not using a piping bag, dollop the icing onto the cakes with a spoon and spread gently with a butter or flat knife.