• Preheat the oven to 180°C (350°F) and line a muffin tray with cupcake cases.
• Beat the butter and caster sugar together.
• Add the eggs one at a time, beating in between each addition.
• Add the vanilla and mix.
• In a separate bowl mix the flour, hot chocolate mix, baking powder, baking soda and salt together.
• Beginning and ending with the flour mixture, alternate adding the flour and milk to the butter mixture.
• Spoon in the mixture into the prepared cupcake cases and bake in the oven for around 15 minutes until the cupcakes spring back to the touch.
• Allow to cool completely before filling and icing.

The Filling

• Place the butter and marshmallows in a saucepan over a low to medium heat. Stir constantly until the marshmallows have melted.
• Fill each cupcake with the mixture.

Buttercream

• Beat the butter and hot chocolate mix until smooth and creamy.
• Add the icing sugar and two tablespoons of milk or water and mix until all combined and smooth.
• Add extra water or milk if the mixture is too thick.