Combine milk and vinegar in a bowl. Let stand until milk curdles, about 5 minutes.
Preheat oven to 180 C / Gas 4. Grease a muffin tin or line with 12 paper cases.
Combine 115g butter, 125g caster sugar and vanilla sugar in a large bowl; beat with an electric mixer until smooth and creamy. Add eggs one at a time, beating well after each addition.
Mix flour, cocoa powder, baking powder, bicarbonate of soda and salt in a bowl. Alternate adding flour mixture and curdled milk to the creamed butter mixture, mixing until well blended. Spoon mixture into the prepared paper cases, filling each 3/4 full.
Bake in the preheated oven until tops spring back when lightly pressed and a skewer inserted in the centre of 1 cupcake comes back clean, about 14 minutes. Cool in the muffin tin for a few minutes, then transfer to a wire rack to cool completely, about 1 hour.
Combine cream cheese and 55g butter in a bowl; beat with an electric mixer until well combined. Mix in vanilla extract. Stir in icing sugar gradually. Make the icing green with a few drops of food colouring.
Place green icing in a piping bag fitted with a grass tip. Hold the piping bag at a 90-degree angle, close to the surface of a cupcake. Squeeze bag to form green 'fur' by pulling tip up and away when the icing strand is about 1cm high. Repeat to cover cupcake evenly with fur. Place 2 sugar eyes on each cupcake.