Preheat the oven to 170C/325F/Gas 3. Line a 12-hole muffin tray with paper cases.
Beat the butter and sugar together until light and creamy. Gradually beat in the eggs until well combined. Sift in the flour and cocoa powder and mix until combined. Fold in the milk, melted chocolate and Irish cream liqueur, if using.
Spoon the mixture into the muffin cases and bake for 20-25 minutes, or until well risen and lightly firm to the touch. Remove the cakes from the tin and set aside to cool on a cooling rack.
For the icing, heat the chocolate and cream in a saucepan over a low heat until the chocolate has melted. Whisk the mixture until smooth and set aside to cool slightly.
To decorate the cupcakes, spread the icing over the top of the cupcakes. Spoon the remaining icing into an icing bag fitted with a small nozzle.
Press a chocolate button onto the cake as a nose, then stick a crisp-coated chocolate on top using a little icing. Stick on two slices of marshmallows as the eyes, and use the black writing icing for the pupils. Stick on the pretzels as the ears.