Preheat oven to Gas Mark 6 / 200℃ / 180℃ Fan / 400℉
Put the eggs, buttermilk and oil into a large jug and whisk until combined.
Add the mashed potatoes to the mixture and whisk together - it will look a bit grainy but don't worry.
Sift the flour, cocoa powder, coffee powder, baking powder and bicarbonate of soda into a large mixing bowl.
Using a spatula stir the wet ingredients into the dry until combined. Don't overmix.
Equally divide the mixture into each muffin case, filling them to about three quarters full.
Bake in the oven for 20 minutes.
Remove from the oven and leave to cool for 10 minutes before turning them out onto a cooling rack.
Make the chocolate fudge glaze.
Heat the butter and chocolate in the milk until just melted. For convenience, I use a bowl in the microwave for approximately 30 seconds to 1 minute.
Add icing sugar and cocoa and whisk together until smooth and leave to cool.
Spoon chocolate glaze over each muffin and decorate with extra chocolate.