For the sponge, measure all the ingredients into a large bowl and whisk until blended and smooth. Spoon the mixture into 12 paper cases, dividing it evenly between them.
Bake in the oven for about 20 minutes or until well risen, golden and springy to the touch. Remove from the oven and leave to cool in the tin for a few minutes before transferring to a wire rack to cool down fully.
To make the icing, measure the butter and icing sugar into a bowl and whisk until light and fluffy. Add 150g of the fudge sauce and whisk to combine.
Spoon into a piping bag and pipe on top of each cupcake. Use a teaspoon to drizzle the remaining fudge sauce over the top.