• Preheat the oven to 325F / 170C. Line a 12-cup muffin pan with paper cases.
• Place the white chocolate into a heatproof bowl, set over a pan of simmering water. Stir occasionally until the chocolate is completely melted and smooth. Don’t allow any water get in the bowl or the chocolate will seize. Remove the bowl from the water and allow to cool.
• In a large bowl, cream the butter and sugar together with an electric mixer until light and fluffy. Add the eggs, one at a time and beating each one in well. Mix in the mashed potato, vanilla and cooled melted chocolate.
• In a separate bowl, whisk the flour, baking powder and salt. Add one-third of the flour to the egg mixture and gently combine, then one-third of the milk. Repeat until all the ingredients are combined.
• Fill the muffin cups three-quarters full and bake for 15-20 minutes, or until a skewer inserted in the middle comes out clean.
• Leave to cool in the pan for 15 minutes, then turn out onto a wire rack to cool completely.

To make the purple potato icing

• Place the white chocolate into a heatproof bowl, set over a pan of simmering water. Stir occasionally until the chocolate is completely melted and smooth. Don’t allow any water get in the bowl or the chocolate will seize. Remove the bowl from the water and allow to cool.
• Beat the mashed sweet potato and white chocolate together until smooth.