Melt the white and dark chocolate in two separate bowls, either over a pan of simmering water, or in a microwave if you prefer.
Leave to cool until it is warm to touch, but not hot (about 35 degrees C if you have a thermometer).
Gently heat the golden syrup to roughly the same temperature (you may need to heat and then leave for a few minutes to cool).
Add 1 heaped tablespoon of the warmed golden syrup to the dark chocolate and beat really well with a rubber spatula until it thickens.
Add the rest of the syrup to the white chocolate, and beat - again, as you beat and it cools down, it will thicken. You may find that the white chocolate thickens more than the dark. Don't worry about this.
Put the chocolate into two separate resealable freezer bags and seal. Place in the fridge for an hour before using.
Cut off small chunks of white modelling chocolate (mine were 10 - 12 g each) and roll in your hands to soften. (If it is rock hard, leave it for 5 to 10 minutes first). Mould into flattened balls to make the faces, then use small amounts of each of the modelling chocolates to make the rest of the features. You can combine a little of the softened dark modelling chocolate with a little of the white to make a lighter colour. The parts should stick together, but if not, melt a little extra chocolate to do this (for such small amounts, I just hold a hot metal spoon against a little chocolate, or fill a mug with boiling water and hold a little chocolate against it). For the pupils in the eyes I used a tiny bit of melted chocolate on the end of a cocktail stick. Set aside (they can be made a day or two in advance and kept in a tin / plastic container).
Make the buttercream by beating together the softened butter with half of the sifted icing sugar. Add 1 tbsp milk or hot water, and the vanilla if using, beat again, then add the rest of the icing sugar and more liquid if needed. Taste and check for consistency and add more butter / sugar / milk or water if needed. Add the food colouring and beat until the buttercream is uniform in colour.
Pipe or spread the buttercream onto the cupcakes and add your Makka Pakka chocolate decoration.