1. Place the cake mix, water and oil into a bowl and mix to form a smooth batter beating for 1-2 minutes.
2. Place into a lined loaf tin.
3. Bake in a lower half of a preheated oven 170 °C (340°F, Gas Mark 3) for approximately 55-60 minutes.
4. Allow to cool completely before using.
5. Crumble the cake either by rubbing with your hands or using a food processor.
6. Sift the icing sugar into a bowl and add the butter and beat until smooth and creamy for about 4-5 minutes with an electric whisk or machine.
7. Melt the chocolate in a heatproof bowl over a pan of simmering water and beat into the buttercream.
8. Add the chocolate buttercream and rum to the cake crumbs and mix well till evenly combined.
9. Squeeze a tablespoon of the mixture together and form a ball. Repeat with the remaining mixture.
10. Refrigerate for 30 minutes until firm.
11. To decorate, dip each rum truffle into the chocolate vermicelli and gently tap to remove any excess.
12. Place into petit four cases.